Its a worn or faulty hinge then, it can be one of 2 things...
1/ When opening and shutting the window, notice there is a black or sometimes grey nylon guide that slides up and down the channel in the hinges, you'll also see on the same guide a tiny little slot headed screw, sometimes the guide snaps right across at this point, its a weak point and with age its a common failure, it leaves half the guide over at one end and the other half moves with the window, this often throws the opener out of line and it looks skew wiff.
2/ If its not that then open the window right out, have a look right in the corner of the hinge and you should see what looks like a 'V' shaped cup, either made out of plastic or even pressed steel, as the hinges scissors together when you close it a little 'V' shaped pointer on the opener locates into this 'V' shaped cup on the frame, this holds the hinge side tight on the rubbers. Sometimes with a worn hinge the 'V' shaped pointer doesn't locate in the cup but tends to sit on the outside of it, this then stops the window shuting tight on to the rubbers creating a gap. With the window nearly shut you should just be able to see if its locating or not, a good test is at this point is to get someone to go outside and to gently thump the corner in, then try shutting it