They see it once, at one speed, from one angle. I’ve seen hundreds of things live at games that I was certain of, only to watch the game back and what I thought I’d seen was not the reality.
I got home after playing Southampton once and I was certain Jay Rodriguez injured himself (that really bad one where he was out for a year) by stepping on the artificial grass just off the pitch. It happened right in front of where I sit… I watched the game back the next day and it happened about eight yards inside the touch line on the pitch.
The officials don’t have get far too much grief in this sport from fans who get to see eight replays from five different angles and in slow motion.