I suspect most other clerks do the same, as when we received instructions for hand stamping the First Day covers for the Reagan/Bush Inauguration, we were specifically made to practice applying even pressure on the envelopes to ensure an even postmark.
Bottom of year gets hammered every time.
Now, if you look at the 8 and compare to the 0, it is worn a bit more. This is because when it is positioned in 2008 the 8 sits closer to the edge.
If we look at the 8, inverted, the right side is worn more than the left. Normal wear for the 2008 insert.