The problem is intermittent and I don't think it has to do with the lockscreen per se. My guess is that it happens when the o/s is subject to other application demands and cannot respond promptly to the slide action. Furthermore, I believe the problem is made worse by repeatedly swiping the lockscreen; that only seems to make the demand on the o/s even worse.
Tedd's Phone Tools and myLock Phone Tools each provide an alternative in the form of a push button over the lockscreen. Each seems to improve the situation, but I haven't found either to be 100% reliable.
Best advice, I think, is not to be too anxious to answer a call. Let it ring, count to three (or five) and then push the button (or swipe the lockscreen.) That seems to (mostly) eliminate the problem.