If it is for a DCPS school, it could be that a child moved "IB" and did not have "IB" preference during the lottery. They jump to where their master puts them in the rankings.
If it is for a charter - a child enrolled and a sibling on the waitlist did not have the right preference assigned to them previously.
Imagine:
Sibling A is #1 on the waitlist for 5th grade.
Sibling B is # 142 on the waitlist for 6th grade.
Sibling B stays where they are until Sibling A enrolls - now they have priority of sibling enrolled and jump from #142 to #1.