It's interesting that ***1 squawks are seen as potential problems, yet others which are also 1 digit away are not, e.g. 7570, 7610, and even 7000 - all of which are likewise 1 digit removed from a disaster squawk! Could it be that we see these as a difference of at least 10 numbers and they therefore don't register as close?
Think I'll make this the basis of my psychology thesis (if I ever study pshychology).