You are always going to see a little variation from shelf to shelf. As long as they are wired correctly and all the heat sources are identical, the thermostat should have no problem regulating them if they are in separate pieces.
I think when I redo my flexwatt I'm going to use a single piece though. I have to resolder my connections ever few years and it gets a little annoying to resolder 7 sets of heat tape.
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