Oh sure, I get that, but I'm referring to, for example, estimates between £150 and £200, say, which finish at £850 hammer. That type of disparity. They are quite frequent.
You can get a bidding war on any given item, which, to me, suggests one of two things. Either the estimate, for whatever reason, is set way too low, or the lot is one which several bidders happen to want and is either rarer than anticipated, or is much rarer in the grade offered, than originally judged.