We have seen this quite often in our allience wars. There are quite high level players that have recently joined the allience we are facing, most often one day ago, that right after the war leaves the allience.
I have searched around the forum and have come to the conclusion that this should not affect the matching since it is based on the players when the matching starts. The top 4 players in this case was above 4100 in power and the most powerful 4400+. The two strongest in our allience is 4200+, the rest is 3500+ and below.
What I cant understand is why people want to do this since, from what I can see, are only drawbacks for the people doing this. Less loot due to lower war partcitipation level etc. The other people in the allience however gain alot since it might be a higher chance for winning, potentially. I have only seen this when facing russian alliances.