Where’s the A????

Hello! I’ve not posted here before and I’d like to sincerely apologise in advance if my question may cause any offence 😉

When us English say the word twat, we pronounce it to rhyme with hat. However, when I’ve heard Americans say it, it’s pronounced twot. How come? What’s wrong with the A??

Incidentally, Jamiroqui (a famous singer here) used to be affectionately known as the Twat in the Hat because of his rather ludicrous headwear!