The Dell OEM industrial grade server fans have a top speed of about 4,000 rpm. So if your Noctua NF-F12 are at their 100% of 1,500 rpm, AWCC will show them as 37% (1500 / 4000 )
Recently I bought 3 Noctua NF-F12s to improve cooling in my system, as well as give it a good clean. I installed one on the front, and two on top for a push-pull on my aio. At idle it is working amazing, giving me a -5 to -10 C decrease in temperatures. Once i loaded up games though, i noticed much more stutters and realized the fans were stuck at 42% for the top, and 39% for the front. Setting an offset or fan curve in AWCC wasn’t doing anything.   
Aurora R10, fixed fan speed