i’d say it is probably done to slightly offset the BCCH multiframe vs the TCH multiframe. Indeed, a MS in dedicated mode (TCH) must listen to neighbour BCCH, which are not synchronized.
If BCCH and TCH are using the same structure, then the MS will never be able to pick up the SCH blocks. Its measurement window will always fall on the same slot in the BCCH.
With TCH=26 and BCCH=51, there will be a gradual offset so that once in a while, the SCH of the BCCH will fall into the measurement window of the MS.
When two equipments are not synchronized, you must ensure they are properly asynchronized 🙂