Why use short-hull and long-hull Perry labels?

Why did the US Navy use short-hull and long-hull descriptions for the Perry class?

The later ships were about two metres longer; batch one and batch two are preferred here because the dimensional difference was small.

The later hull was about two metres longer, which explains the short-hull and long-hull descriptions. Batch one and batch two are preferred here because the change was less than two percent of the original length. This naming question is separate from the aviation reasons for lengthening the design. The inconsistent use of Flight I and Flight III remains unresolved.