The reason is the current B12 schools have been promised the opportunity to play TX and OU before they leave for the SEC.
If you say there are four more schools in the conference from the P12 in 2024 then there are more conference games. If you need to insure the current B12 teams play Texas and OK then you need to load that up in 2023 to make sure those who don’t play in 2023 are able to still play in 2024 even with the new conference members.