As someone who used to do editing, including in the game industry its astonishing what will slip past you. I've had things I must have read a dozen times that I suddenly spotted when the actual book arrived. Some of it is that since the work is often a moving target, you come to expect the part you were used to seeing, even when the author(s) change it. The other part is telling you you'll get to looking for that stuff once you've dealt with the big stuff (missing text, format problems, so on) but by then you've seen it so much you sort of don't see it any more.
This is one reason medical and legal texts usually have a raft of editors and proofers, to protect against that, but game products can't usually afford that.
As to spotting it in playtesting--sometimes there isn't that much of it (especially in the crucial blindtesting) and sometimes the playtesters just get focused on other things.