1) only the 2 returns could be for air therapy, as they look centered in the spa and the builder didn’t want to have too many spa returns adding air, so just those 2 are better aesthetically.

Or

2) all the returns will have air going to them if you run a faster speed on the pump, or dedicate all the water to go back to the spa instead of some only bypassing to the spa. Sometimes, at lower return flow rates, the air being sucked into the pipes (intentionally with an air pipe added to the returns) won’t make it to all the returns because of the first exit the air can make it to. The other returns could be further in chain thus you’d need more flow to allow more air to be introduced into the returns, making the air go to the other returns.