In answer to the original part of this post, there is not much difference at all. Reason being that they have made extensive changes to the way that Matchmaking and difficulty bonus points are scored. Initially you are matched with opponent defences which are similar in power to the defence that you yourself put up. You then move up & down based on a) if you win or lose and b) how easily you win or lose.
The victory points seem to be pretty well fixed, the difficulty bonus points are based on two things. 1) how powerful your defence is in the overall heirarchy of the tournament and b) how well your defence has previously performed (i.e. an E ranking gives less bonus points compared to an identical defence on an A ranking).
So you are right that the best tactic is to always aim for an A defence but you also don’t gain anything by setting a weak defence to try manipulating the enemies that you face as you a) will receive less points for wins and b) will end up scaling quickly if you are wiping enemy teams.
(Apologies for previous post @Scarecrow, I misread some of what you said 
)
Sources for information:
- 🏅 [Second Raid Tournament] Discussion and Feedback on the Updated Raid Tournament Matchmaking & Difficulty Bonus
- 🏅 [Third Raid Tournament] Discussion and Feedback on the Updated Raid Tournament Matchmaking & Difficulty Bonus
- 🏅 [Fourth Raid Tournament] Discussion and Feedback on the Updated Raid Tournament Matchmaking