During my last playthrough, I noticed the Replica has quite a few weird things about him. Lets start with a short explanation. Based on what I can see, the fights go in this order.

Riku I > Sora I > Sora II > Sora III > Sora IV > Riku II

Here is where things get weird. Repliku during Riku 1 is actually stronger then during Sora 1 and 2. This is found by the fact he can do Helm Split and his attack is higher then normal. That means he randomly became weaker after his first battle and took 2 fights with Sora to regain it.

This happens again at the end in a very unrealistic location. In his 4th fight with Sora, he is actually stronger then the 2nd fight is Riku. This makes no sense at all. First off, how did he even get those "God Powers" for the Sora fight? He didn't absorb Zexion until shortly before the 2nd Riku fight. Plus, in the context of actual power, absorbing Zexion actually made him weaker! He has 7 1/2 health bars against Sora but only 6 1/2 against Riku, plus he does less damage against Riku. I've actually survived a Dark Aura with Riku on Proud before whereas I always die with Sora.

Long story short, the Replica randomly gets weaker in random locations. There is a possible reason behind the first loss of power but the 2nd one doesn't.