Because you operated on it with a function and so it is NOT the same variable any more -- that is documented behavior. varfun doesn't know (or care) that the function only selects a given subset of the same variable and doesn't modify it. It ((varfun, that is) also doesn't try to parse the function name (since so often one uses anonymous functions amongst other reasons) to add to the variable name; it just uses 'Fun_' as the arbitrary identifier.
You can manually rename the variables if you wish, of course...
T2.Properties.VariableNames=T.Properties.VariableNames; % put original names back
