I'm in the process of re-reading all the books in order. I just got done with the 3rd.
In the 3rd, it states that Fred & George managed to get a few O.W.L.s and Percy did wonderful on his N.E.W.T.s. This was written while everyone was still at Hogwarts, before summer break had started.
In the 5th book, it states that Harry and everyone else will have to wait until the summer to receive their O.W.L. results.
Why the difference? Do students normally get results before the end of term, but because of the Umbridge issue, was there a delay?