I think it is a confidence thing, if I haven't flown for a couple of weeks I get quite bad on the RT. It only takes a couple of calls to your local ATC before you are back in the swing.
I must say that I don't think the standard is that bad (this probably means that I DON'T sound like an off-duty 744 skip.) I think the worst seems to be London information on 124.6 but that seems to be mostly due to traffic calling them and not hearing the controller reply. (It always amazes me that someone will call 124.6 without first waiting to hear the controller, do they think that 124.6 is just very quiet today). The times I have had to relay are ...well once.