@variablepulserate You mean there are people who don't?
I have wondered the same, but the other way round. I think tor-toys might be more northern and tortus southern. I've always said the later, but I had non-northern parents.
According to t'interwebs it comes from Latin (tortuca), so tortus is more appropriate a pronunciation. I did wonder if there was a French influence, given the -oise ending, which would have meant tor-twahs would be more accurate. However, it apparently got that ending from a 16th century spelling "tortoyse". It rarely pays to assume where words derive from, though I find it a fascinating topic.