If I'm not mistaken, it is only because the boot sequence is power consuming. If you wait a couple minutes and check again the time left on your tablet, it will go back to near 5 hours, maybe 4, depending of how much time it was turned off and at what point exactly it was in its power content.

I usually check the number of days left if inactive to know at about what percentage my tablet's battery is at. I found out that after a charge (100%), it is around 10 days. From my testings, it seems that 8 days left is around 80% left and so on. I would like to know if a percentage-showing battery indicator exists so I can check if I'm right.

Anyway, is there a reason for you to shut down your tablet? From what I've seen here and on my tablet, keeping it on (and "locked") is the best way to go as it consumes about no power on that state, even with wifi on. I can let it sit in my pocket all day long at school and I've got no problem power-wise as I never ran out of juice.