24 GB isn't a "normal" amount of RAM, like 4, 8, 16, 32 GB are. So that's one possible cause. The other possibility is BIOS simply can't report a number higher than 8 GB which was the upper OEM spec... Since it's working, don't worry...  
I have been playing around with my old Inspiron 570 turning it into a "shop" PC. I installed two 8gb sticks of 1600 dual rank RAM in Dimms 3 & 4 and two 4gb sticks of 1600 single rank RAM in Dimms 1 & 2 just to see what would happen. The BIOS says there is only 8gb of installed RAM and I suspect that is because it is only designed for 8gb. In windows, (Win 10 64 bit) It shows 24gb installed and says nothing about only XXgb available. CPU-Z and HWINFO also both report that there is 24gb installed and running dual-channel, unganged. Task-manager also reports 24gb of ram. So, am I to assume that there is indeed 24gb of installed ram or is the BIOS correct that there is only 8gb installed??? 
24 Gb RAM installed on Inspiron 570