While using Shattering Rune the count does not update. It only updates when you log out of the game entirely and then start it back up. It is possible to display like 70 runes and not be able to shatter because you have run out.
To Duplicate - Grab ~10 items you want to shatter. Note your rune count. Shatter items. Rune count still the same. Buy some runes from the vendor. Rune count still the same. Run a monolith to get more gear to shatter. Rune count remains the same. Exit game and come back in. Rune count changes.