I do not think the matchmaking is taking boosts into account. Or if it is, they are not weighed heavily enough.
I see the same thing in our alliance, some have moved way up and others way down.
Just a thought here. It could also be how they decided to boost. Perhaps users who put their boosts into one or two have a better chance of drawing them so will advance faster? Those that spread the boosts out are not as strong so have advanced less. Those that have not purchased boosts, use only the daily incubator ones, are moving down the ranks. Or perhaps those with the most speed boosts are advancing faster?
I chose not to boost anything, as an experiment. I wanted to see where a solid 4100 - 4200 team would end up. Currently just below 3600 and am able to get many 3-2 battles now.