I've noticed several times that the loot count being displayed is off. It corrects itself, but sometimes it shows that the amount is say 10/10 C, when you are actually only 8/10.

This was implemented uniformly across all the addons as part of commit ced3e06

Oh, I have a solution for this for one of my bag management addons.
You need to delay processing of the event till 100-200 msec after the last BAG/LOOT event fires.

http://www.wowace.com/addons/ace3/pages/api/ace-bucket-3-0/

I've noticed this as well. Will have to look into it.

I've seen something similar in Parrot for it's counts when I loot something. According to the Parrot FAQ, it has to do with the fact that the event for looting sometimes fires before the item is in your bags and sometimes after, making the stackcount off.

From the Parrot FAQ (http://wow.curse.com/downloads/wow-addons/details/parrot.aspx)

Q: "Sometimes when Looting stackable items, the stackcount is off. Why isn't this fixed?"
A: The problem is, that the Interface-event for the Loot is sometimes triggered before, and sometimes after the item was put in the bag (depending on lag). 
If anyone knows a solution that does not involve constantly polling the bags' content, I'm open to suggestions.

Grrrr, I thought it always fired after. That would be the issue. I'll still look into it and see if I can fix it, maybe making a temporary function that checks the stack count a moment later.