Didn't think of searching about the CPU's support. Then that answers it. ASUS should fix their specs.
Technically the RAM is 2400MHz RAM as you showed in the CPU-Z screenshot, it's just your exact specification has the lower end CPU which only supports 2133MHz RAM so the RAM is being ran slower for the CPU. If you had one of the higher end CPU's available in that laptop it would have ran the RAM at 2400MHz. I agree it is a little confusing and misleading but it would be more confusing to the general public for ASUS to try distinguish all the different combinations of specifications.