Potion uses and charge dont match

I have noticed that while playing through, I am short on uses. The flask I am currently using is the giant life flask, with no total charge increase or charge cost reduction. It states I have 75 total charges, and each use costs 10 charges. When I use the well to fill up, that flask has five uses. What am I missing? Also, my mana flask has a capacity for 99 charges, which cost 10 per use, but the maximum uses are 6.