power level changes, anything to worry about??

I am not experiencing any issues, but I was wondering about the changing power levels and if it indicates a upcoming problem. I started service last Sept/Oct, back then the power levels varied for the 8 channels between "-0.3 to -3.5ish". I've noticed it changing gradually over the months to "-10.0 to -7.0s". are these levels affected by outside temp perhaps? (las vegas heat). anyway, I recently just upgraded to preferred service and notice the levels in the "-12.0 to -9.0s". nothing has changed on this end equipment-wise since I hooked it up. I used the provided 2way splitter and cables that came with the dpc3825. Again, I am not experiencing any connection issues but was wondering if this is indicating something might be failing. what is the optimal specs/range for it? or am I worrying for nothing? thanks.