Arena

Haha why are you so surprised of winning more games? Its normal when you 2 lost 9-1 last week... your team rating would be brought down and thus competing with the teams at the bracket what (which means equally lousy)

Arena operates in a very interesting way

You fight with teams within your bracket. eg. 1200-1299 teams fight each other.
However it is possible you'll meet those 1700s or 1800s teams too. If you win against these teams your rating increases a lot and if you lose you only lose a small amount because they are SUPPOSED to beat you.

<1500 there's a formula to calculate it
>1500 there's another to calculate them.

Generally, with the same team rating 5v5 > 3v3 > 2v2 in terms of points. (3v3 is 80% of 5v5 points and 2v2 is 70%)

Lastly, arena is only open for lv70s and many say that S2 stuff = T5 equipments & S3 = T6. People say that because the graphics or how the gears look like are identical between these 2 set of gears. If its really equal, you think people will spend time grinding boring instances for T5s/T6s? Arena gears are generally high in stam, crit, resilience but low on damage and hit.