I know I'm a dumba$$ sometimes, and when I see something that seems to be impossible, it more than likely that I wasn't paying attention or I was being careless.
It happens, but your readings being that far off I would have to say "there were other forces at work", that made it inaccurate either the first or even the second time.
IMO your first week reading is probably an inaccuracy, I just brewed a stout that was 1.060 and it took over 3 weeks to ferment out. I don't know what kind of yeast you are using either, but it may just take a little longer because of the higher OG. If it is an extract brew with specialty grains, there will be some unfermentables in there as well. My OG 1.060 wouldn't drop below 1.018, and that was after 4 weeks (28 days) in he primary, I racked, primed and bottled from there.
Best brew yet...