I'm an ESL, mind you but I think the vowel gets pretty cramped with similar short vowels (hem-ham-hum) so the tendency is to make it more different. It drifted

I'm an ESL, mind you but I think the vowel gets pretty cramped with similar short vowels (hem-ham-hum) so the tendency is to make it more different. It drifted away in general in American English, but the vowel in "air" stands in the way in non-rhotic accents, so the change is much more random and word specific.