Hi!
In broad lines, the "a" sound is pronounced differently depending on the position of the tongue (front - at, central-asleep, and back -all). In a word like "anatomy", the sound of the vowel "a" is not the same just because it looks the same. Can you guess the position of your tongue in pronouncing "a" in this word?
Anyway, these different sounds of the same letter (a) are called allophones of the /a/ phoneme.
There are even situations when the sound "a" is diphtongized, in different Am dialects or even when stressed: " this is a wonderful oportunity." I'm more familiar with the AmE.
But of course, there are differences in pronounciation between Am, British, Australian, Canadian English and so on, as there are differences within these languages (the Southern drawl, for instance, in the US, is different from North American phonetics. It comes from the lengthening or the diphtongization of the vowels, "a" included.)
Check out this link if you want to see the mechanism of pronouncing the "a" sound.
Phonetics: The sounds of American English