Because as you said, it's a mix. My 535, before its blank screen issue (which is not related to the "mainstream" bug of touchscreen), had no "ghost typing", etc. Sometimes, it missed when I typed, sometimes, it clicked instead of scrolling, but every once in a while, and never to the point to get frustrated.
I believe that a SW fix was enough for my device. But for the first batch, which may have OTHER issues, like the keyboard having a life of its own, only a return to a Nokia Care will do it according to me. And as it's not systematic, Microsoft have not called ALL 535 back.
I hope my repaired 535 or swap device is not buggy ; my original one was OK, I'll discover it today or tomorrow.