AFN, Low idle speed during rollout

AFN in Golf III, 1997, 256 tkm, Schaltgetriebe, modifiziert vom Vorbesitzer.

VCDS shows me a constant idle speed of 903 rpm when the car is stationary and idling. There have never been any deviations in this regard, except when the engine was very cold.
However, sometimes when driving, it can happen that when releasing the clutch without selecting a gear, the idle speed is lower. Then, as soon as the car comes to a stop, it returns to the normal level. Climate and large consumers were not factors in these cases.

I haven't logged this phenomenon yet, but I can rely on my ears (and the tachometer also shows it).

Just out of curiosity: What could be behind it?

Hello Klaus

[Translating...]

Hallo,

das Verhalten ist "Serie".

Viele Grüße, Rainer

[Translating...]

Hi,

bei neueren TDI hat man dieses Verhalten uebrigens "invertiert". Mein CFHF laeuft beim ausgekuppelten Rollen mit etwas unter 900 U/min und faellt im Moment des Stillstands auf knapp ueber 750 U/min ab. Sobald das Auto (aus welchen Gruenden auch immer) rollt, geht der Leerlauf wieder auf den hoeheren Wert.

Viele Gruesse,

Jan