That's just naturally how a engine works. If you look at the bhp and torque against revs. Working from torque x revs / 5252 = bhp

So, if your torque kept climbing with revs then so would power, but. Your engine might make 200bhp at 6000rpm so torque has to fall with revs.

Hope this isn't confusing as its late and I'm half asleep

You also need to look at boost, as with turbo engines it's easy more boost = more torque, therefore power.

Soy our peak torque may have been at higher boost than the higher rpm points making the drop of torque more apparent.

In basics it's just how it is and nothing to worry about

Jim