Compact frames with sloping top tubes often have 2 measurements. First is the nominal size, like you measured. There may also be a virtual size which is the equivalent size if the top tube were horizontal. Always ask for lots of different measurements when buying a frame without test riding it first. For most people, the top tube length is more important than the seat tube length.

Also, Italian bikes are often measured to the center of the top tube instead of to the top of the top tube. That can cause a 1cm difference in nominal size.