Also, there is no onscreen evidence to suggest that the Mirandas existed in the time before TMP. In fact, if the Enterprise refit was a testbed for a bunch of new technology and ideas (as seems to be the case), then it's very likely the Mirandas were entirely new ships grown out of that leap.
That would put their origin around 2280 rather than around 2245 or so.
I always assumed that this was the case. Miranda was a mass production model, Connie-refit was basically an expensive prototype built on older hull.