The following is an example of how leveling and improving skills works:

1 x Unit + 1 x Leader goes Scouting.

Significant Success is rolled.

You gain 1 XP for Skills
You gain 1 XP for Leveling.

You can assign the XP for the skill to either the Leader or the Unit.  It can be in either Scout or Spy (because you were Scouting).  You reply with who gets it and in which skill.

You can assign the XP for the level to only the Unit (Leaders cannot level-up.)  You then reply with the stat-increase.