Limited data rate still allow higher conn. rate?

RouterOS Wireless Networking
1

I have an AP (ros 4.16nv2 package, normal 802.11a mode) where I set supported rates A/G to 6, 18 and 24Mb.
Associated nanostation5 has data rate set to 24Mb with autofall back enabled.

MT AP still shows Tx Rate = 54Mbps? How is this possible?