That's because the welsh y sound is slightly further at the back of the throat. Linguistically, there's not a lot of difference between YA and IA, just like there's not really a lot of difference, in your mouth, between WAA and OOAA. W is a vowel in welsh, the vowel-consonant designation is a pretty arbirary one that differs per language.

Sorry, my background is actually in linguistics and phonetics, I do go on sometimes.