Does it stay like that for a while? Over time the charge from the dock will discharge and into the tab and charge it.
If I leave mine docked for a while the tab will be 100% and the dock 0%
The way the battery system works is that one battery charges the next. When you dock the keyboard it's more akin to plugging in something like a power pack. The battery in the keyboard will charge the tablet. If you plug in the keyboard then the wall charges the keyboard which charges the tablet.
If you look in the bottom right corner sometimes you'll see a little double arrow moving from one battery to the next. That's indicating that the dock is discharging and the tab is charging. Sometimes I don't see that so that makes me think it doesn't charge continuously.