DDR means double data rate -- the speed quoted is twice the actual clock speed.  What you're seeing is normal.  
I own the xps 15 9520 with the i9, 32gb ram. The stats on HWInfo64 show 2x16gb ram modules being ddr5 4800, so I understand their speed should be 4800 MHz, however the monitoring shows a max clock speed of 2400 MHz. I wonder if anybody else experiencing this discrepancies...seems to me like a harware malfunction of some sort but wanted to make sure before contacting Dell for warranty claim. Thanks in advance for any hints about this
Max Ram speed of 2400 MHz on XPS 15 9520 i9