I’ve noted when I up the intensity that initially my HR climbs but after a while it can drop even though the intensity hasn’t. It’s almost as though it takes a short while to get to the optimum operating conditions when it comes to oxygen demands of the muscle.
I wonder if it’s to do with muscle motor unit recruitment. That as you spend longer at an intensity the motor unit recruitment changes. If fresh units are brought in to replace or supplement fatigued ones that oxygen consumption requirements go down. So in the ramp test although more power is being demanded it changes the motor unit firing and thus oxygen demand. Thus they can make more optimal use of units available from the oxygen that can be supplied.
You can imagine a signal from brain to muscles going “Right this is as much oxygen as you’re going to get, so I’m going to change things a little to eek just that little bit more power for a bit longer”.