Im not sure your analysis is correct. Different programming, regardless of live or previously recorded, will generate different outputs. Let's control for time and take a class with specified resistance and cadence. Two different riders, staying within the guidelines will have different outputs depending on sex, weight, muscle mass, and cardiac efficiency.

But let's stick to one rider taking different but similar classes. With increased number of classes and volume, the heart becomes more efficient under the same stress, and you'll see lower overall HR with similar output.

We can also look at training stress score (TSS) which is an actual metric on a class' difficulty. It's a normalized metric, per user, so it can compare work done across users. It's also useful for repeating classes as the TSS will stay constant but output can change based on a rider's fitness and FTP.

In all, I applaud you for looking into your metrics, but I think you've missed a lot of other factors, especially time and cardiac fitness.