The problem there is with catch up EXP in the jungle.
By you clearing the jungle, you level up and bring the level of the jungle up (jungle monsters spawn with the average level of all players in the game). By having smite and a jungle item, you can get bonus exp from clearing monster camps that are higher level than you. And, you get 30 x level difference per monster in the camp, so Krugs and Raptors offer a ton of exp when they are higher level than you. Krugs (10 monsters) can give you 300 bonus EXP from being 1 level behind them. I think the level difference stops at 7, meaning Krugs could give you 2100 bonus EXP if you were 7 levels behind them. That could be 2 levels from one camp depending on what point you were in the game. Of course, if you're some how 7 levels behind and the enemy hasn't ended the game yet, I'd say that's the enemies fault for not ending with that much of a lead.
This is why some junglers start Raptors, because since the jungle monsters start at level 2, you get that bonus exp from the camp, giving you level 2 plus like 1/6 exp towards level 3. Plus they spawn a few seconds before the buffs do, so on blue side you could do raptors with help from mid plus your AOE, get level 2, and still have bot leash your red buff without putting them behind.