The problem with trying to “ramp up” on a dynojet is, the bearings for the drum are extremely low resistance.
It takes much less energy to turn the drums at driving speeds than it does to push the car down the road at those same speeds.
So, if you gradually ramp up to wot, by the time your foot hits the floor the rollers might be going 50-60mph.
This loads the converter in a way the will provide lower numbers, because there is less speed differential between the engine and input shaft(less tq multiplication), as well as allowing the motor to be at an even higher rpm.
This is similar to trying to do the high gear stall test while going 50-60mph.
The observed flash stall speed will be higher and you’re not going to get an accurate picture of the true stall speed.

The only real “load” you get from a dynojet is when trying to change the speed of the drum.
You don’t get to ramp up to wot against a predetermined start rpm like you would on a dyno with a load cell.

With a high stall converter, the longer it takes and the higher the speed of the drum before you get to wot and pushing hard against the weight of the drums........ the smaller the window of usable data there will be on the sheet.