Strange thing i dont understand. Maybe someone can explain...

i put my miners at Kano for about a week, and pulled them temporary to an other pool to generate some coins.
Naturally, my hash graph slowly decreases.
By the time first block January hits, i get paid for 166Th
Next block: 153 and then 152Th
I added all my miners again, and my graph is 130-ish.

Since i added my 235Th, my graph is still declining for the past 2 days?
Yesterday was 118 average, now it is 102??? With 2 days miners running already?

Anyone care to explain?