Bosdur is right, and tootee is right. The problem here is that attack/defend (or melee/defend if you wish) have to be integers. Remember that this number appears in the exponent, and raising a base number to a real exponential is very computationally intensive.

(1 - 2)/2 = -.5 means no bonus.
(2 - 2)/2 = 0.0 means no bonus.
(3 - 2)/2 = 0.5 means no bonus.
(4 - 2)/2 = 1.0 means 1 point bonus.
(5 - 2)/2 = 1.5 means 1 point bonus.
(6 - 2)/2 = 2.0 means 2 point bonus.
(7 - 2)/2 = 2.5 means 2 point bonus.
(8 - 2)/2 = 3.0 means 3 point bonus.
(9 - 2)/2 = 3.5 means 3 point bonus.

So on the even armor upgrade transitions above 3, an armor piercer picks up 1 point of attack, but looses it back to the +1 defense for no net gain. On the odd transitions, an armor piercer looses 1 net combat point like any non-armor piercer.

I am assuming here that LongJohn fixed the way armor piercing works to be consistent with his original intent as described by him here at the org and in the Strategy Guide. In MTW v1.0, armor had to be greater than 5 for armor piercing weapons to get a bonus. That was a late change prior to the original release suggested by Activision, and LongJohn subsequently said that he didn't think the weakening of the armor piercing was a good change.