which means he could have seen Sheva was 1 metre ahead or 1 metre behind... seeing as he signaled offside...
Are you a ref? have you ever attended ref classes? how many matches have you watched?
It's not that rare, though I admit is uncommon for the ref to ignore the assistant, but it can happen
here you go:
http://www.fifa.com/en/laws/Laws6_01.htm
as it says, their duties are subject to the decision of the referee...
it also says, if you browse the rules, that the referee "has full authority to enforce the Laws of the Game"
Even Milan fans have stopped crying over this...