マイケルさん、
I think the pronunciation of borrowed words are usually based on US English pronunciation (キャ for "ca" and so on)
because of English education in Japan. After WW2, English education was of course all about American English, so
the pronunciation taught at schools were also mainly American. This "tendency" is still true in most of the schools.
On word basis, it really depends on "when" and "how" the subject objects are introduced. If you know the Japanese
words related to car, for example, many of them are rather based on UK English.
i.g. ウィンカー、ボンネット、ナンバープレート
Of course, for some reasons, we adopted US words too...
Interesting observation would be the word "can" (= "tin" in UK).
I think Japanese never really recognise this word as キャン simply because original Japanese actually is 缶(かん)
Natsuko(奈津子),
Team JapanesePod101.com