I am building a dark library. I began with 10-min exposures at -15C. All seemed fine. Now that I am doing 15-min exposures (also at -15C). The 15-min exposures seem a little brighter. All variables are the same except for the exposure lengths.

I use SGP and the mean ADU value for the 10-min frames is 491, whereas the mean ADU value for the 15-min frames is 502. See below for two examples. Is it normal that the 15-min exposures are a little brighter?