Yup! Most banks participate in only one card network clearing cycle per day.
This is where Mastercard or Visa receive files from everybody connected up describing every transaction they want to collect/send money for, process them, distribute files back out, and work out how much money is owed to/by each participant.
On the Mastercard network, there are around 3–6 of these cycles per day (except on Sundays) so that every time zone has one after the close of the business day. Monzo participates in all of them to provide the smallest amount of time we can between a transaction finalisation or refund being submitted through Mastercard and us receiving it.
It also helps us on the processing side to have smaller files more frequently instead of one big file per day that needs to be processed quickly.