I get the same thing with my own name...very frequently...and it is annoying. My name is Shannon (Sha-nin) and I get that "OH, you mean Shah-nun". I think it's because As are often pronounced differently between Americans and Brits. We tend to just say a whereas Brits seem to pronounce it as ah.
Don't get me started on my name.
My name is Marcia, pronounced Mar-sha.
In the US, if someone say my name written down, they would ask me "Is that pronounced Mar-sha or Mar-see-a?", because the name, as it is spelled, can be pronounced either way. I would say "I prounce it Mar-sha", and they would call me "Marsha" from then on. Simple.
In the UK, if I tell someone my name is "Mars-ha", they will call me "Mar-see-a" anyway. Sometimes when they haven't even seen it in writing. Even people who I have known a long time, whom I am sure have heard me pronounce my name more than once.
I can mention my name, "Mar-sha", and a minute later someone who just heard me say it will call me "Mar-see-a".
I've even been asked WHY I call myself "Mar-sha". - Because that's what my parents named me

I can understand if you've only seen my name written down and pronounce it the other way because you haven't heard how I pronounce my name.
But I don't understand why someone would pronounce my name differently after I've told them how I pronounce it.
I just give up and let people call me "Mar-see-a" because it's just too difficult to deal with.