From UFC RULES AND REGULATIONS:
UNIFIED RULES AND OTHER MMA REGULATIONS (emphasis added)
Except with the approval of the Commission, or its executive director, the classes for mixed martial arts contests or exhibitions and the weights for each class shall be:
Strawweight up to 115 pounds
Flyweight over 115 pounds to 125
Bantamweight over 125 to 135 pounds
Women's Bantamweight over 125 to 135 pounds
Featherweight over 135 to 145 pounds
Lightweight over 145 to 155 pounds
Welterweight over 155 to 170 pounds
Middleweight over 170 to 185 pounds
Light Heavyweight over 185 to 205 pounds
Heavyweight over 205 to 265 pounds
Super Heavyweight over 265 pounds
In non-championship fights, there shall be allowed a 1 pound weigh allowance. In championship fights, the participants must weigh no more than that permitted for the relevant weight division.
The Commission may also approve catch weight bouts, subject to their review and discretion. For example, the Commission may still decide to allow the contest the maximum weight allowed is 177 pounds if it feels that the contest would still be fair, safe and competitive.
In addition, if one athlete weighs 264 pounds while the opponent weighs 267 pounds, the Commission may still decide to allow the contest if it determines that the contest would still be fair, safe and competitive in spite of the fact that the two contestants technically weighed in differing weight classes.
I don't know why you are thinking some fighters are below the weight limit. For example,
Champ Cris Cyborg (145) vs. Holly Holm (144) – for women’s featherweight title
Holly Holm is inside the weight limit of Featherweight (over 135 to 145 pounds)
Michal Oleksiejczuk (203.5) vs. Khalil Rountree (205)
Michal Oleksiejczuk is also inside the weight limit of Light Heavyweight (over 185 to 205 pounds)
Mark De La Rosa (135.5) vs. Tim Elliott (135)
Mark De La Rosa is above the weight limit of Bantamweight (over 125 to 135 pounds) and not below the limit but because of 1 pound weigh allowance (non-championship fights) is allowed to fight.
So for your question,
When making weights, why are some MMA fighters sometimes below the weight limit?
They are not below the weight limit, while some of them are above but because of 1 pound weigh allowance for non-championship fight they are allowed to compete.