[Elecraft] K2 filter center frequency shift with band?

[Chris Kantarjiev]

I'm playing in the RTTY Roundup this weekend, using a simple AFSK setup
with my K2. For some reason, when operating on 15m I need to shift
the Mark frequency to get the signals to fall inside the filter.

This doesn't match up with how I understand the K2's filters to work;
I expect the center frequency of the filter to be the same regardless
of the band.

What am I misunderstanding?

73 de chris K6DBG