The bug

When you use the QueryResult stat in combination with the /xp command, the query result value is the previous value. This affects adding / removing experience and adding / removing levels.

How to reproduce

1. /scoreboard objectives add xpLevels dummy
2. /scoreboard objectives setdisplay sidebar xpLevels
3. /scoreboard players set @p xpLevels 0
4. /stats entity @p set QueryResult @p xpLevels
5. /xp 1L
   → The score has your previous xp level count as value

The reason

The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.

The reason for this is that the method net.minecraft.command.CommandXP.execute(MinecraftServer, ICommandSender, String[]) first sets the query result value and then changes the experience value.