you know the effects options, the built in ones? reverb, delay, fruity filter etc? if your using the " enhanced stereo", that could be the problem. or it could be another effect. thing is, do you have any effects running with that synth?
i could be soooo many things..... for a start, theres fooking loads of pan switches in your average audio route.... 2(?) in the sampler, then there could be some in whichever vst's are on that channel, then the one on the mixer..... and then it goes to the master where theres more!!
also, like others have said, could be a stereo sample with only one channel of audio
or possibly something wrong with your hardware??? dodgy connection, dodgy lead, dirty knobs faders on mixer/amp, family of borrowers inside one of your speakers... who knows??