According to his own account, Daniel Tammet changed his original surname of Corney to Tammet because he did not like his original surname while he did like the surname Tammet, which apparently has a meaning in Estonian that Daniel liked. I find it interesting that the surname "Tammet" is a name that was once assumed by an Estonian man who liked to change his name and make improbable claims. I've got to wonder, had Daniel been reading about impostors around the time that he chose his new surname?
Wikipedia contributors (accessed 2011) Alexei Tammet-Romanov. Wikipedia, The Free Encyclopedia.
Impostor Pretenders. General Books LLC, 2010.