More than a ranking list
It would be simple to always match the next available fighters in strict ranking order, but matchmakers also weigh how recently each fighter has competed, whether a style matchup is likely to produce a competitive fight, and how thin or deep the division is at that moment. A purely rank-based system would sometimes force uncompetitive or repeatedly cancelled matchups, which is why deviations from strict order are common rather than exceptional.
Styles get considered too
Two fighters can be closely ranked and still make for a poor matchup if their styles are too similar to produce a clear result, or too mismatched to be competitive at all. Promotions weigh this because a series of unwatchable or lopsided fights is bad for the division's credibility even if each individual pairing was defensible on paper by ranking alone.
Depth changes the calculus
In a division with many ranked contenders, matchmakers have room to be selective and can build meaningful eliminator fights. In a shallow division, the same standards would leave cards unbookable, so compromises such as rematches or fights against out-of-division opponents become necessary. Judging any single matchmaking decision without accounting for the depth of that specific division misses most of the actual reasoning behind it.