I don't think this is a valid assumption. That is certainly not the way it is presented. It is presented as a way to accurately determine training zones to achieve specific results. Polar doesn't tell you that the zones built into their watches are just suggestions that may or may not put you into the training zone indicated. And as as I've said, the guidelines may just as well be too high as too low. This is from Polar's website:

"Getting the most out of your training doesn’t always mean working faster or harder. The best way to improving and seeing results is to train at the right intensity. With a Polar heart rate monitor, you can make sure you’re not over or under training, and make each session count. Use Polar as your wrist-based personal trainer, motivating you each and every time you train.

Calculate Your Maximum Heart Rate
220 - Your Age = Maximum heart rate. For example, a 30 year old’s maximum heart rate = 220 - 30 = 190
Level 3 moderate range = 190 x 70-80% = 133-152bpm"

I don't see anything saying this is just a rough guideline that errs on the side of safety.



That's fine; I have no issue with that. I'm just saying that if someone is going to base their training around these numbers, they should be accurate and they should have a clear understanding of what they mean.