Anime series are counted in cours, blocks of roughly a dozen episodes corresponding to a broadcast quarter. The unit shapes almost everything about how these shows are produced and scheduled.
The quarter is a television scheduling unit
Japanese broadcast schedules are organised into seasonal quarters, and a slot is committed for that period. A series is commissioned to fill one.
Sponsorship and advertising are sold against the same period, so the financial arrangement matches the broadcast one exactly.
Everything downstream follows from that alignment. The production is planned, financed and delivered as a quarter of a year of television, and even series that never touch a broadcast slot are now built to the same shape.
Production capacity fits the unit
A studio has a limited number of experienced staff, and a cour is roughly what a team can produce at acceptable quality within a planned schedule.
Committing to two consecutive cours doubles the exposure without doubling the available people, which is where schedules break down.
Splitting the order gives a studio a gap to recover, complete outstanding work and prepare the second block properly. Without it, quality degrades visibly across the later episodes as the schedule tightens.
Source material has to stay ahead
Many series adapt an ongoing manga, and an adaptation that catches up with its source has nowhere to go.
A split allows the source to advance during the gap, so the second block has finished material to work from.
The alternative is original content written to fill time, which historically produced the least popular stretches of long-running series.
Split scheduling manages commercial risk
Production committees fund these series jointly, and a split lets them observe reception before committing fully to a continuation.
A first cour that performs poorly can be concluded, while a strong one justifies the second block and the merchandise, music and home video releases planned around it.
The structure is therefore a staged investment as much as a production schedule. Announcing two cours in advance signals confidence, and announcing one keeps the decision open at very little cost.
What the gap costs
Audience attention does not hold cleanly across a break, and a series returning after a season away has to rebuild the conversation it had.
Simulcast viewers in other territories feel this more sharply, since the surrounding discussion disperses entirely during the gap.
Studios and platforms manage it with recap episodes and compilation films, which are cheap to produce and exist mainly to keep a title present during the months it is not airing.