Presumably if a particular ship design fills a mission role extremely well, or flexibly enough - then it keeps being constructed.
Excelsiors and Mirandas seem to fit in a wider range of roles, even outside of light cruiser and heavy cruiser brackets (with the Miranda becoming closer to a destroyer escort over time) - so apparently, they kept building more. In widely spaced batches.
Conversely, the Ambassadors seem all bunched together, whether by preassigned number blocks or not. They fill a cruiser role - but only during a specific technology peak era. Hence they probably had a shorter production span.
And the Niagara just seems to have been built piecemeal, every so often.
Excelsiors and Mirandas seem to fit in a wider range of roles, even outside of light cruiser and heavy cruiser brackets (with the Miranda becoming closer to a destroyer escort over time) - so apparently, they kept building more. In widely spaced batches.
Conversely, the Ambassadors seem all bunched together, whether by preassigned number blocks or not. They fill a cruiser role - but only during a specific technology peak era. Hence they probably had a shorter production span.
And the Niagara just seems to have been built piecemeal, every so often.