[Elecraft] Question about filters in USB as shown on a P3

[tomb18 at videotron.ca]

Hi,

I have a question concerning what you see on the P3 when you tune in a USB signal. 
The green band that represents the bandwidth is not quite at the tuned in frequency (center frequency) but is displaced slight higher in frequency. If you lower the bandwidth say using a 2.1 KHz filter it has shifted quite a bit up.
Can someone explain what is happening from the radio point of view please?

Thanks, Tom VA2FSQ