Also XP gain is scaled, so that the harder it is (ie, lower % chance of success), you get more xp.
If you have a 50% chance you get the xp listed on the chart.
If you have < 50% chance you get more xp; if you have > 50% you get less xp.
At least that's how I understand it.
SO getting 6xp from a 10xp item probably means you had a more than a 50% chance of success.
Not a bug, just how it works.
