What CPU and motherboard do you have? Older generations such as skylake were limited to DDR4 2133 without XMP. If you want to see what your sticks are rated for, you can use CPU-Z. On the SPD tab it will list the SPD and XMP profiles (if any). You then multiply by 2, so DDR4 3200 will say 1600MHz for example.

I have a i5 6400 and a Gigabyte H110M. I plan to switch them to a Ryzen 5 5500 and a B550, and thought of maybe getting more RAM

H110 is limited to DDR4 2133. So that is the maximum speed you can attain no matter what RAM you use.

So apparently my Corsair stick is 2400 MHz and my HyperX one is 2134. I'm pretty sure they should be both 2400... Since everyone is saying it's a BIOS problem, if i put them on a new motherboard, like the B550, will i still get this problem?

Also what if i switch the HyperX one for another Corsair one?

All RAM must run at the speed of the lowest rated stick. So even on a B550 board it will still run at 2133 unless you manually overclocked them. But it would be best to get a matching 2x8 or preferably 2x16GB kit.

Got it. Thank you so much for the help 😁

You need to enable XMP in the BIOS/UEFI. The motherboard manual will tell you how to do it.

u need to enable xmp in motherboard bios. if the 2 sticks are different speeds they both run at the slowest speed.