Maybe an example would be helpful. Some of the values will have to be made up of course.
Under the old system the match is based on a delta calculation. Two alliances would be matched if they were within some fixed delta of each other’s O/D. If Team 1 had O/D rating 4500 and Team 2 had O/D rating 5000, then their delta would be 500 and if the matchmaking threshold was 500, then the two teams would be matched.
Under the new system, the match range is a percent delta calculation. In the above case, the % delta between the two teams would be 500 / 5000, or 10%. Based on the OP, the new system does not consider 10% to be a good match and the two teams would not be matched.
So under the old system that had a fixed delta range for matching, it was more likely that teams would be mismatched when they were of lower ages because their O/D would be lower. Under the new system, the matchmaking range scales up or down with the O/D level, making it more likely to get a fair match at whatever O/D level you are.
My thought is that they would have calibrated the new % delta range for matching so that it would not impact top levels much, and it’s impact would be most felt at lower O/D levels.