2.5TD AT rev counter needle

[Driver+Passengers]

I've driven two Bongos, and the rev counter behaved differently in each.

In mine, I'll drive along at 40 or 50mph in top gear, and approaching a junction, back off the gas - right off. The rev counter needle drops to the same angle as the speedo, and then they drop down together - always the same angle. Until the speed is 0 and engine is idling at 800 or whatever. Alternatively, if still travelling at speed I go back on the gas, the needle will rise from 'same angle as the speedo' to 'wherever it bites' to give me power again, and I can ping pong it from actual speed to 'same angle as the speedo' by going on/off the gas.

The other one I drove would drop right down to 'idle' at 800 after backing off in top gear, and up to 'wherever it bites' to give me power again.

Though I drive an auto daily, I'm confused. What is the transmission actually doing when I back off? Is one or other clutch at fault, or have I got my wires crossed? (Literally?) If the wheels are turning the engine, surely I wouldn't be flip-flopping between two different rpm values if the auto remains engaged. If it disengages when I back off, then I'd expect it to drop to idling speed, which it what it sounds like its doing. The fact that the angles of the needles seem identical suggests to me that they might be getting the same voltage, or something, but then I don't really understand how they work, yet.

Can anyone shed some light on this for me?