Some terminology might help first.
To me, the HST pedal position is not "Throttle". The HST pedal is more of a continuously variable gear shift. The more you push the pedal, the higher the gear ratio.
That lever that you move to change RPM's is not exactly the throttle, either. Its actually a governer setpoint. It sets the desired RPM's. The governer mechanism constantly adjusts the actual throttle position to try and maintain the set RPM's.
The actual throttle is the thing that controls how much air and fuel goes into the engine. Its usually a butterfly plate in the carburator. As explained above, it is controlled by the governer.
Whenever the governer has the throttle at any position other than full open and is still able to maintain the set RPM's, the engine is not putting out full power.
Whenever the governer has the throttle all the way open, its because it cannot maintain the set RPM's at any less throttle position.
If you put the setpoint at a lower RPM and then load the engine down until the governer has the throttle full open, its the same situation as if you had the setpoint at full RPM's and then loaded the engine down until it reached full throttle and lugged down to the lower RPM's. Either way, you are at full throttle at some less than max RPM.
Another issue - all governer mechanisms have some slight droop. They have to drop a few RPM's to get the throttle to open. So, under load, it is normal for the RPM's to drop slightly. This does not mean the engine is "lugging down" - its just normal governer droop. You can easily notice this when you run the RPM's up to PTO speed and then engage a PTO implement like a finish mower. The RPM's drop maybe 50 to 100 or so.
Now lets throw the HST pedal back into the mix. At a given setpoint on the governer, we will have some throttle position to produce enough power to maintain the set RPM's. We now depress the pedal, increasing the gear ratio, and thus increasing the torque requirement on the engine. To maintain RPM's, the governer opens the throttle. So, the HST pedal does indirectly affect the throttle, but it is not a direct relationship.
Sometimes, the HST pedal effects the engine RPM's very little. Rolling at constant speed on a flat, hard surface, there is very little additional power required to roll along at 8 MPH instead of 3 MPH.
Pulling a heavy implement up a hill, or mowing through thick grass, the increase in HST pedal ratio and thus ground speed can make a huge difference in power required. At that higher "ratio", the engine lugs down. Pressing the pedal just cranks up the ratio even higher, making the problem even worse when the right answer is to let up on the pdel, shifting to a lower gear ratio to climb that hill and maintain engine rpm's with the governer just kissing full-open throttle.
The JD load-match system attempts to control the HST ratio to make it more like a throttle pedal in some cicumstances. by backing off the ratio when the throttle hits full open.
It would be even cooler if the pedal itself had some feedback - maybe resistance to being pushed in proportion to throttle position with a hard stop when the actual throttle maxed out.
