I've bought two pair of brawling gloves today (mining) to buff my total xp to its limit. Both have explicitly shown that they offer 750 charges, but when I started mining my first rock (runite), it suddenly dropped to 500. My first assumption was that it was just some glitch, but when I started using my second pair of gloves, I ended up with the same result.

Not sure if this also happens to occur with other gloves; might just check on those as well.