99% of cars lean off at high revs.
Way they are mapped. They run daft rich when booted at lower revs then as the revs increase they lean off, and off and off a bit more.
Ever watched a car in front especially a big one like a new jag or something boot it and the puff of black smoke? Thats full load enrichment which is fine there but it leans off and off.
All maps are a comprimise. It may be lean on one car and the car built 2 misn later may be running rich at the same load, speed etc.
get the car on the rollers and make sure the loads held for long enough and watch the AFR.
Problem with most RRs are they dont hold the speed long enough to actually highlight these probs.
Also stock ecus lean off at certain points to comply with emmisions, noise regs and the fuel economy figures.
Its a bloody hard job mapping a road car ecu, thats why manufacturers spend a wee bit of cash.