thats the way it works. most rigs will get more in OD. leastwise the ones with decent power, unlike the vette, which got the best rwhp in 2nd....
one thing is the turbo spools better with a load on it and it gets a better load in OD.
every rig is different. pays to do one run in direct so ya know, but figure most of our rigs cept for my isuzu and sometimes the 6.2's will get the most out of OD.