That is a pretty significant increase! Over 2 1/2 times base.
I googled Testapel and found the typical dosage is 150mg - 450mg every 3-6 months, but usually every 3-4 months. Each pellet has 75mg of T. So you must be getting 900mg. Just doing some rough math and you would be getting approx. 900/105 days = 8.6mg T/ day.
That's interesting you didn't get much jump in your total T on 10 pellets (750mg) but it shot up hugely with 12 pellets. Your story is very encouraging!