Ram wont post in slot 1 and 3 @ 3200MHz, but does in slot 2 and 4

[baapcgaming]

Im using 2x8GB G.Skill TridentZ 3200MHz (16-18-18-38 1.35V) and my pc wont post at 3200MHz in slot 1 and 3, but it will in slot 2 and 4. PC will only post @ 3000MHz in slot 1 and 3, so what is causing this?

Specs:
i7-8086K
Asrock z370 extreme4
EVGA GTX 1080 Ti
8x2GB Ram (gskill tridentZ)
EVGA G3 750W supernova
NZXT H500 case

 

[drivinfast247]

That's normal. Look at page 17 of the manual.

 

[baapcgaming]

That's normal. Look at page 17 of the manual.

Why though? It just shows a chart about ram population slots.
Sorry if this sounds dumb, but thanks for the help!