Good question. Heart rate drift will also be limited on the high-end, which is what the half-marathon reveals. Once you’re near your maximum for a given duration, there isn’t much room to increase. Judging by some generic formulas (which are never perfect), an average HR for a half-marathon around ~1h30′ is typically ~95% of anaerobic threshold. So your AnT HR might be around 180.
But the aerobic threshold drift test is on the lower end of the spectrum. It’s a place where there is lots of possible room to increase, but due to the intensity being within your aerobic capacity, it doesn’t.
In between the two is Zone 3, where there is room for HR to increase, and it does. Right up until it reaches AnT (or a little higher than your half-marathon HR).
Does that make sense? In the AnT case, HR has no room to move (at that duration and intensity), so it doesn’t. For AeT, there is room to move, but the load isn’t enough to make it happen. Your aerobic capacity can comfortably deal with it.