How did it come to be that the main course of a meal is referred to as "entree", where that french word has always meant "appetizer" ? Did someone translate it incorrectly it ages ago, and we haven't caught on yet, or is there another reason?
We're applying a French word to a Russian practice. This can only go wrong!
More seriously, we're not even doing that, but the dining practice at most high-end Western restaurants that don't highlight the following of a particular ethnic practice is a late-20th Century adaptation of service à la russe.
Now, the fact that the English term for this is to use a French term that means "service as the Russians do it", is a potted history of this approach all in itself. Indeed, generalise that to "the English term is a French term that means how they do it outside of Western Europe" and you have a story that has repeated itself several times in the history of haute cuisine.
Service à la russe means that you have several courses, which are brought to the table as the previous one is finished, or ideally after just the right length of pause (a skill that sets a Michelin star quality meal apart from a merely excellent one, in itself). But really it means more than that, and what we have is the result of over a hundred years of changes to that style of dining, that is changing still as innovations from the far ends of the spectrum from the likes of Noma and the Fat Duck on one end, and fast-food establishments on the other, work their way into the middle. For a start, we most often eat only 2-3 courses with maybe a couple more on special occasions and even a 3-hour tasting menu may have less than a dozen courses (I don't recall one going above 12 including amuse-bouche and petit fours) where originally service à la russe would have frequently had 13, 14 or more courses.
Before then, we had service à la française which was also sometimes called service à l'anglaise, but which wasn't really called anything until the French and English wanted a name to mean "the way we used to do it, before this new fancy Russian way became fashionable". This is close to a buffet style, but with some items brought to and from the table (what the Scandinavians still call "cold table" is close, if generally less showy).
Most of the food would be laid out on the table in a manner designed to impress on first sight, with a roast ready to carve part-way through the meal, but some would be brought to the diners, and taken from them when they were finished.
These dishes would be brought in with some fanfare, of which the best modern example (indeed, one that often goes beyond how it would have been in those days) being the piping of the haggis such as I would have enjoyed on Friday had their been any justice in the world and I hadn't been too busy to celebrate Burns Nicht properly. That is to say, they would have made an entrance, or in French, une entrée.
Even at this time, just which dish (or perhaps, just which entrance) counted as the entrée wasn't a point of universal agreement, which possibly owes more to hosts' desire to place the focus on a dish other than the first, that they were particularly proud of leading to the term being applied to a course after the soup and/or fish, than to either of those.
So, in using entrée at all, we're using a term of imprecise meaning, for something at some point in a particular type of service, while practising an adaptation of yet another type of service, in which we have much fewer courses, and in which those courses are nowhere near as well defined in terms of contents or sequence.
Really not all that surprising that we don't agree on what it actually means, is it?
I suspect that confusion over just what does and doesn't count as an entremet didn't help matters either (strictly a small dish between more substantial dishes, but it's an even older word being applied to an even more recent course, so it can be even less well-defined).
The French word doesn't actually mean "appetizer"; it comes from a dish introduced between two courses in a formal dinner. It may originally have come from the word "entrance" and even changed in its French meaning.
OED has a citation
OED also has a "chiefly North American" variant, given as "the main dish or course of a meal", and supported by another citation:
Thus it's still a dish introduced between two courses, but rather than a light dish it's the main course.
I hate to submit a simple link as an answer, but this does seem to cover your question http://www.dailywritingtips.com/what-is-an-entree/
I also don't like simply copying and pasting the content of an answer from elsewhere -- yes, this does make my submission liable to staleness of the link but copying someone else's work seems somewhat unfair.
EDIT (June 10 2015)