Because of their previous wins and loss it just so happens that their MMR is in that range, its not that complicated. More games played does not mean you have a higher or better MMR then any one with less games. Look bud, If you assume the system functions in a very basic manner its not surprising to see matches like this. Grab players from pool in a range to build games, does not inherently make for the best balanced teams because it very often does not have players MMR set at a point that actually reflects their skill. A basic system does not have safety nets in place to prevent a mismatch, it simply care about large numbers of games and getting to a point its collected enough games on any one player that it can be self correcting in their placement of mmr to skill. Your going to get lemon games, your gonna get mismatched games, its gonna be both ways.
If its desperate enough to be grabbing grossly mismatched players because a lack of options... then it would/should be entirely capable of distributing them equally between the two teams. It cant be a basic system if it intentionally loads uneven teams. a on paper 1300 elo vs on paper 1800 elo team would never fly in a simple system.
I took the time to plug in every player in the site you listed.
You, normal 1,582
Lux normal 1,624
Vi. 1,559
Xayah 1,616
Diana 1,617
Zac, Vayne, Nidalee, and Sion are a group, All of them have a pretty low # Normals played.
Like I said Grouping up does really weird things to MMR gains. Veigars the odd man out but he ques with hommies to much for the site to find his MMR as well.
They all pretty much only play with friends and very rarely do they do normal, I think its reasonable to assume that their MMR after going thru group modifications is in the 1600 range. I don't think they would stay in that range if they play seriously and grinded out more games.
Normal inherently are valued less for balanced teams, blind pick, large groups, its more for fun then to be a competitive match. Stuff like this just happens, nothing personal, nothing evil or intentional, just a uncaring system dicking you over with the way it built teams and not giving a dam about individual matches outside of grabbing from the available pool near the set team MMR.