Aero road bike frame - frame size number vs actual frame size?
The answer is the top tube is not straight but curves down at the seat stem.

The idea behind the difference is that somebody who need a 49cm size frame would have the seat adjusted to their leg length and somebody who as a slightly smaller leg length can't lower the seat to suit. So you have got 10 mm up and 10 mm down otherwise your seat would be set at the top of the seat tube all the time. Same on the next size if you bought  a 49cm frame but your leg length is that bit longer you buy the next size up and lower the seat to the top of the seat tub.

You have a 49cm frame with +/-10mm, and the next size frame is a 51cm with plus and minus 10mm. Plus you can work out your crank length from 165 mm to 175 mm to suit your leg length.

So this mean you have some 40 mm to play with in various leg lengths. And the same with your cage length. Or top tube as some people say. If you've got short arms you use a small handle bar stem or long arms a longer handlebar stem.

I hope this may give some of you some better idea and thought to buying your frame. Measure your leg length and subtract 30mm from that and work from there.

I know this as a fact as I design cranks for a living. Hope you have something of a better idea now.