[Elecraft] Is this normal?

[KJ3D]

This is probably a stupid question - the answer to which I should probably
know - but I just can't see it.

I have (I think) properly adjusted my K2 (4991) using Spectogram.

If I tune in a CW signal with the mode switch in U or L, and then mode to C,
the signal shifts about 600 Hz.  

Why would the frequency where the signal appears to be shift?

TIA,

Tom, KJ3D 
Curious in Virginia