know why the rise and set times do not change syme

I've noticed that the rise and set times do not change symmetrically. For instance: for Honor, Mi for the dates 1/1/15 thru 1/11/15 :
Sunrise remains at 8:21 on the 1st thru the 8th
In that same period sunset occurs later by 7 min.
From the 8th to the 15th rise gets earlier by only 2 min, while sunset gets later by 8 min.

I'd have thought that (except for rounding) the geometry of the situation would cause the rise and set changes to be equal (and of course opposite)
Can you explain?