This is not really about national or cultural differences.
99.9% is an over-used figure of speech. It gives the impression of being a true statistic, when it is more a ploy of advertisement. We've been inundated with infomercials that sell us on ideas by using high numbers to convince us that we should buy things to be safer, prettier or healthier. Basically, anything above 90% is seemingly good, as it means top rate or ACE. Does that truly make it more valueable?
Since the issues are about the point being made, the person you are trying to convince of this point or both, the statistic itself is fodder. People with analytical minds will question what you are saying when you use a statistic.
If the point is far-fetched to the listener, the listener questions either your statistic or questions the validity of your point as having no proof of the numbers. The supposition is that in using 99.9 percent, whatever follows is a gross exaggeration. Using such a statistic harms credibility. When you use unproven numbers as proof of truth, by default what you are trying to prove is discredited.
If what you are saying is well-received or agreed upon, there would be no contention. It just may be that more Americans simply do not really agree with your point, not so much the statistic itself. They will try to make sense of what you are saying, or will have a difficult time believing you at all.
And what of that .1%? Some people are skeptical about that .1%. By focusing on that .1%, they will will challenge your opinion. Since, some people love competition when meeting people from other cultures, this would definitely be an example of people being obtuse.
Percentages have more credibility when the speaker has a profession where quoting statistics is a normal behavior and fact based. Random percentages are opinions. This is reminiscent of when people say "Everybody does it" or "Always" or "Never". These are overstated absolutes.
Try dropping the percentage and just making the statement and see how it goes?