Everyone learns pronunciation from someone else, whether it is their friends, their family, or the media. The most common American and British pronunciations you listed are generally accepted in those countries by consensus. I am sure if you told people in either country that you were looking for a "JA-gwar," you would appear to be the odd one.
I agree with the "tomato, tomato" post from earlier.
Simple accent differences, no offense to the animal. The 'a' follows a 'u' and makes it kind of difficult for some people. I say "jag-warr" and even that is met with a raised eyebrow! I once heard "jag-oo-warr" which I scoffed at.
English speakers are separated by a common language.