as a presumption, i would think that the chip is basically an interceptor for your fueling map. Increasing the mixture within the cars limits to make it go better...
I have heard of something like this on other cars, where because the fuel map has changed to push more fuel through when it returns to idle the fueling decreases slower than it used to. If it isn't running lumpy i would guess the maps working right, but the way the revs increase and decrease will have changed.
my car does the same thing, but slightly different mate. If im driving it fast, when i return to idling revs the car will sit at 1300rpm, until i set off again or dip the clutch and roll a little, then it returns to somewhere below 1000rpm.
I've had this thoroughly checked out, and there's nothing broken or anything. I wouldn't worry about it.
