I'll spell it out for you..

Yes, the when the flap is fully open you require additional enrichment. So lets say at 1.2 bar of boost you require X amount of enrichment to maintain a wanted AFR.. Then you wind it up to 1.4 bar of boost (on a turbo that is more than happy to supply the air flow) but the AFR doesn't change...

If it was flowing more air, then it would go lean until you added more enrichment..