simple words:
COMPUTER ESTA STUUPIDO!
the AFDS (auto flight director system) and the FCC (flight control computer) have a programming. if the DME is closing to zero and then counts up again it knows it is flying over the VORDME.
and then there seems to be another programm which tells the computer if you loose the signal (no DME!) and you are still high (5.000ft +) then you can not be in an approach.....