833 is not a theoretical speed, but a channel speed. In this case, the data rate is usually 1/2. 420mbps is the actual data rate for this device.
(*) … interface rates are not the net data rate. There are a lot of mandatory inter-transmission gaps in 802.11 wifi , The higher the interface rate , the higher the relative impact of those GAPS.
Small transmission blocks (A-MPDU) , make that impact (lossed air-time) very important. The MT WLAN driver has limited transmission block sizes. The new wifi drivers don’t have that size limitation.
There are free spreadsheets to calculate that inter-gap impact : http://forum.mikrotik.com/t/help-mikrotik-capsman-wireless-download-speed-max-200mb-but-pcs-mobiles-link-speed-is-866mbps/142677/1
http://forum.mikrotik.com/t/wifiwave2-expected-performance/160075/1