Why, after a quick lookup at the Google online library, allowing the browsing of the content of available books, can I inevitably find instances prior to the first known use in the dictionary of certain words?
Because the fist known use of a word is exactly that: the first known use. If and when they find an antecedent, they will usually update the dictionary entry.
There's no way for them to search every single book and document ever published, and not every publication still exists. However, more documents are being computerised and indexed so this makes it easier to find antecedents.
The OED are very happy to receive antedatings for inclusion in future editions, as well as new meanings. Check their FAQ to find out how to contribute.