I would absolutely be concerned about the advertised frame size. Since when is it OK to misrepresent items for sale?
If a shop sold a 58 frame as a 56 then they either lied and therefor committed fraud or they made an honest mistake. Either way, if it is not a 56 then they need to make it right, return the money or get a 56 frame.
Specifications are "only a guide" but it doesn't mean that manufacturers and retailers can make false claims.
If it is a 56 frame and it doesn't fit or it is a 58 frame and it does fit then the OP has a problem with what he thinks his size is.
-Tim-