It occurs to me that all
the Nebula class ships might be precursors of the Galaxy class, the Galaxys are a follow on class, with the Nebula construction being discontinued prior to the construct of the first Galaxy.
(Yes, I had to quote the ginormous smilie.)
I like it. This is probably close to true - I mean, apparently the Galaxy
s were in development hell for a while, so maybe the Nebula
s were meant to prove the concept and technology behind them, as well as fill in a support role once the Galaxy
s came online. Thus, they'd already all be built and in service by early in the Galaxy
production run if not before.