I am trying to track the current xp value (not levels) of a player by using a scoreboard objective: xpCurrent. I use /execute @a ~ ~ ~ xp 0 @p in a fill clock to query the xp value of all players and store it to the scoreboard with /stats entity @a set QueryResult @p xpCurrent. This works until I manually adjust the levels of a player with xp 1L/-1L @p or use the enchanting table. At this point it will display the total amount of XP the player has collected. The only way to reset the counter is to kill the player or remove more levels than the player currently has.

Code analysis by marcono1234 can be found in this comment.