U can't compare two bikes based in stand over, the top tube could be higher in the smaller bike and lower in the bigger one, then based in that logic u decide to go with an extra size of the bigger one because it match what you know and end up with a frame 2 sizes bigger. Hypothetical case but clearly you are looking a 2 cms frame that apparently has maybe the same/close stand over than a 56, what that tells you? Bingo, geometry is different, BB in the 58 for sure is lower than in the other one u know or whatever u use.

Use stand over to figure it out but in general thats the reason u have reach, top tube size, and seat tube size, and always carry a METRIC measuring tape, metric error is about +-1 mm.

Agree with campy also.