The sizes vary from 80s to 90s frames.
In the 80s, top tubes were horizontal. In the 90s the sloped downward. Measurements are still done center-to-center on both, but an 18" frame from 1995 is much larger than an 18" frame from 1985. Look at the top tube lengths to better compare the frame sizes.
For example, I'm 6'1" and ride a 21" Fat frame from the 80s. For the 90s sizing a M/L fits me. A M/L is probably an 18 inch frame?