XP system broken?

I am genuinly wondering what I did wrong in the following game. Or what my team did wrong in the game.

The matchup Us: Top: Nocturne Mid: Kassadin Jungle: Evelynn Bot: Ezreal Sup: Janna

Enemy Bot: Xin Zhao Mid: Karma Jungle: Sejuani Bot: Kog maw Support: Nautilus

I am playing Evelynn and manage to get firstblood on the enemy Sejuani. Nice start! Then I see Nocturne which is obviously a bad top pick having trouble versus Xin Zhao, so I decide to pay top a visit. Nocturne commits and dies in my gank but I manage to kill Xin, 2/0 for me. However I have fallen behind a little in xp because I have ganked twice so early.

Kassadin makes 1/0 against Kog Maw, but Kog picks up a kill against Janna. Xin picks up a kill top versus Nocturne and is now 2/0. Nocturne tp's and kills Xin. Janna accidently kills Kog Maw, irrelevant but bot lane is winning hard. Kassadin and I manage to get another kill bringing me to 3/0. However Xin also gets another kill top. 3/0 Xin.

At this point I have to decide, am I going to help top or am I going to try and snowball other lanes. I decide to help other lanes because top might not be viable to gank anymore. Xin is now level 9 and Nocturne is stuck at level 6 and Xin also has a 50 CS farm lead. Kassadin picks up another kill putting him at 3/0 and I pick up 2 kills putting me at 5/0. However somewhat later Xin gets a double kill on Kassadin and Nocturne.

The state of the game Xin 5/0 Me 5/0 Kassadin 3/1 Others Irrelevant.

However, Xin is level 13, while I am level 9. I have fallen behind in XP because I have not been able to farm like Xin. I am on an equal level with Ezreal, and a level behind on Kassadin. Meanwhile Nocturne and Janna are behind a lot. We are still ahead in kills though and our team except for top is much better, but Xin has become unstoppable. He pushed the top turret and is now pushing for the 2nd turret and we try to stop him, but not with any succes. He can now 3 or 4 shot most of us, because he is 4 levels ahead and has a huge CS lead. The game starts to fall apart and we simply can't kill Xin anymore, he even manages to kill us 1 v 3. In the end Xin is level 15 and I am level 10.. 5 levels behind.. So is Ez and Janna and Nocturne are even behind 6 levels on Xin.

Now you would say maybe Xin is somekind of LCS Pro or at least a Diamond smurf(this was a gold 3 game) but not even close. this Xin has 3 wins and 7 losses. A KDA of 10/9/7. Yet this player can destroy the whole game, while I was just as fed as he was, but he had the opportunity to farm and I did not, because I am a early game jungle assasin that could not stop this guy. Our whole team was ahead, we had all lanes but top won easily, yet this top laner can solo this game with his level advantage. I yet have to find the balance in this.

You can argue that their team had a much better late game, but Nautilus and Sejuani weren't even to be found in any team fights, they weren't even relevant to the game. Xin could do it all alone. How can a single top laner be so far ahead on a full team even if he managed to free farm most of the time with all those kills?

Why does he deserve to be so much stronger than me when we are both fed, especially when I am supposed to be the early game monster when fed? How can I win a game like this? Sorry about all the ranting, but it's annoying the fck out of me that I lose games like this. I mean 5 levels, what is that about.