Why do rent amounts vary so much on multi-family?

[Aaron Wyssmann]

Not real experienced when looking at multi's so this is new ground.  So I've been looking through the rent rolls on several properties all in the same town and not too far apart.  They are all similar in size and built around the same time frame.  What I'd like to know is why the rent amounts vary so much.  

For example, in four 4-unit buildings this is what the rents look like. 

585, 610, 615, 585, 585, 615, 630, 585

585, 615, 630, 585, 585, 615, 630, 585

I'm guessing that some of this variation is due to supply and demand (winter and summer) but are there any other logical reasons as to why this would vary $45 between the high and the low?  These numbers look very much the same on other couple of buildings I've looked at.  So  in doing my analysis on this deal, I'm assuming that I'd base my numbers on the lowest rent of $585 for all 16 units and then any extra would just be gravy?