It depends on how many arena points your opponent has. 

Those with more points award you more points if you win, and you lose fewer points if you lose. 

If you fight those with fewer points than you, you win fewer points if you win, and lose more if you lose. 

Same rule applies to Tag Arena as well. 

Look at your arena rating and the opponents, then it will make sense.  If they are roughly the same it will be 10 pts.  For teams lower rated than u it will be <10... Higher will be >10

It's pretty straightforward