i believe it's a situation where the torque converter is locked up vs not locked up.
with the torque converter slipping (i.e., in non-locked-up mode), the slippage is converted to torque, hence the term torque converter. This happens at high load, and your engine RPM will be higher than usual for a given gear and vehicle speed. The less viscous the transmission oil (e.g. at higher temps), the more slippage you can expect.
As the load decreases, the slippage decreases, and the torque converter approaches lockup mode, and your speed increases with engine RPM constant, in the same gear, until the lockup point.