How do level 120 items level?

What if the EXP gain on Reverse/Timeless equips are based on using a certain skill?

Lets take the knuckle for example. It has a chance of giving +1 to skillID 5121007 at MAX level according to Fiel's thread which is Barrage I believe? What if you would only gain EXP for the knuckle when you use Barrage?

 Spoiler

리버스 에퀴녹스
Pirate only - Knuckle
Req Level: 120
Req STR: 330
Req DEX: 120
Weapon Attack: +82

Level-up system
EXP Table: 70 --> 75 --> 80 --> 0
Bonuses each level: Weapon Attack(1-2), STR(1-2), DEX(0-1)

You have a 1 in 10 chance of getting a skill at level 4
Possible skills: 5121007 +1

It's seems like a much more valid explanation to the idea of 1 EXP point per skill usage.