The 'Next Level' in the inventory shows the amount of experience you need to reach the next level, rather than the total experience required; as you gain experience it will decrease until you level. You need 8,000 experience to reach level 3, for example, so that should be the sum of 'Experience' and 'Next Level' as long as your characters are on level 2.
Mouse over the experience bar at the bottom of the screen to see the experience past the current level and total experience from the current level to the next one.
You need 2,000 experience to reach level 2, for example, so when you are on that level, the bar will show (your experience - 2000) / 6000.
