There looks to be only three of the large turbines, and a half dozen much smaller ones? Maybe the small ones came on line for load leveling being much easier to spin up and down than one of the big fellas? The small ones could also possibly have all been bought on line when a large one was down for maintenance? Probably materials technology could have limited the turbine/alternator efficiency, no rare earth strontium magnets around in the 50's and bearings were probably still pretty basic as well.
