Rating Reward Points?

The points are based on the distance between your score in this PvP tournament and theirs, not the roster. If you have equal points, you'll win 25 points for beating the opposing team. If your score is high above theirs, this will be lower (all the way down to zero), if far below, it'll be higher (all the way up to 50).

You're seeing the available points from an opponent drop because your score is rising as you fight more battles, while the points shown for an opponent is based on their score at the time they popped up in your queue -- but I think the actual points you get for beating them may come from their current score, which may be different.

I haven't looked at the maths on maximising your score, but it may be worth just skipping a low-point opponent and hoping the next is better.

Also, keep in mind that a full encounter (offense + defense on retaliation) equals 50 points.

If you pick on an easy target that is worth 15 points to you, that target will see you as a 35 point retaliation. Stay away from the low point fights (below 20) if you can, as the opponent will see you as a juicy retaliation (unless your roster is so overwhelmingly strong that you are sure that the opponent will skip out of the retaliation).

If someone attacks you, you get a red dot instead of a yellow attack node on your tourney screen. That's a retaliation.