It is very hard to tell from the pictures what is exactly moving - the bottom in, or the top out.
The top rails really do not do much to support the pool. The stabilizer channel on the top of the pool wall and the plates on top of the legs are what ties the whole thing together.
Looking at that picture again, it looks like the top is moving out. If, check how the two stabilizer channels (curved metal pieces that go over the top edge of the pool wall) are joined. Some just but together, and others slip over each other, but in either case, the joint should be under the top cap of the leg.
If the joint is not under the top cap, then it is not stable, and it will bow out as you fill the pool.
As me about how the legs on my pool are not evenly spaced, and therefore my top rails need some "creative" bold hole patterns in them, yet none of my walls bow. (I did not install that pool - I just live with the nightmare)
-dave