hifi engine

Quick question for you electronics gurus!

I have 2 perfectly healthy Adcom GFA 565's. They both perform exactly as they should sonically. The question I have is related to DC offset. When I first turn them on from being cold the DC offset readings are as follows:

After 1 minute... left-40 Mv. Right- 2 Mv
After 15 minutes... left- 15 Mv. Right- .5 Mv
After 1/2 hour..... left- 9Mv. Right- 0 Mv
After 1 hour...... left- 1.1 Mv. Right- .1 Mv

I've made these same checks several times and the numbers repeat themselves fairly close each time. Id like to know if there is any sensible reason why one amp stabilizes the DC offset right away and the other takes an hour. I do realize that these are all acceptable numbers. Once they reach their lowest DC numbers, they both stay low and very stable as long as they are left on. Thanks for any input on this. Thanks