[Elecraft] K2: Dial calibration question

[Jim KO5V]

My new K2/100 is working well, so I decided to do a complete realignment and button it up. 

I set up the reference oscillator using Spectrogram and EA3ADV's method. Using 10MHz WWV (and a couple of iterations), I managed to get the frequency to read within 10Hz on 10MHz WWV and 5Mhz WWV and within 10-20Hz on 15MHz WWV (all higher in frequency). However, 20Mhz WWV is off by 80-90Hz lower in frequency - the direction of the error is as expected, but not its magnitude. 

I would expect the error to increase as the frequency increases, but this seems like a lot. Is this another "Don't worry, be happy" situation, or should I look for a problem?

Also, the temperature of the rig and room do affect this! With the lid off, I put a towel over the open top for about an hour, then removed it and ~15 minutes later, the dial had drifted about 20Hz. 

Thanks again!

73, Jim KO5V