The suface book uses the base battery for the majority of the drain while using as a laptop but then as the base battery gets low it starts to use more of the tablet battery while still using some of the base battery. If you used the laptop all day you must have depleted just about all of the base battery causing the system to use the tablet battery to keep running. When you detached the tablet and had 30 percent that was all that was left of the tablet battery and im sure most of the base battery was depleted.
When on battery if you right click the power icon in the taskbar the battery %'s will display for both the batteries (keyboard section and in clipboard). The it gives you an estimated battery life in hours and minutes. I'have never right clicked when the clipboard wasn't attached.
So if you have been using it on battery only with it attached the battery won't be 100%. As you use it attached both batteries %'s go down.