This is how I understand it.
When they were researching into the pictograms to use in place of words on future signs they tried out lots of different pictures.
The ones we ended up with were more readily identified with their purpose than any of the alternatives put forward. That does not mean to say that they unambiguously put their message across. Far from it. Typically somewhere around a half to two thirds of the people shown the signs (the ones we have now) recognised what their intent was. The alternatives faired even worse.
So the designated purpose of each sign has always been somewhat out of sync with the way it is interpreted in practice.
Coming on to specifics, the "briskly perambulating person" (running man) symbol with the downward pointing arrow was designated as meaning that there was a downward change of level on the escape route from that point.
But...
Ever since the introduction of the pictogram signs people have interpreted this sign as being the one to put above a fire exit (whether it be final or otherwise) to indicate that this is the door to go through.
Now, as I said, it wasn't designated as this, but that is how most people have interpreted it when they have installed the signs.
It is therefore wholly reasonable to suppose that the majority of people, when looking for an exit, will interpret the sign in the same way as those who installed it (irrespective of the designated intentions).
At the end of the day, it doesn't matter that much. Everyone goes out the way they came in anyway. But if they are desperate enough to seek an alternative, they will just look for a green sign with anything on it. It could say "M & S Foodhall" and they would still follow it! I know I would.
Stu