Buying on actuals vs pro forma

[Barri Griffiths]

I've always heard and believed that you should only buy on actuals. But I've been hearing more and more of the experts buying at a low cap rate(5-5.5 and even lower)

Therefore I'm assuming they're buying on what the property can achieve in the future. 

There is a property that I've been looking at that have rents that are $100-125 below market.

 And I believe I could comfortably get $75 more per door without having to do any improvements.

A lot  of the tenants are month to month, so I could implement this pretty quickly.

How are these experts buying at such a low cap, and still giving their investor a good rate of return?

And is it unreasonable for me to buy on what I believe I can get the NOI to pretty quickly?