Why were the British divisions not all the same size?

Why were the van, centre, and rear divided into 12, 13, and 11 ships instead of twelve each?

Some sources do give twelve each, but the source used here gives 12-13-11 and the uneven distribution can fit the intended battle plan.

The table used in the lecture gives the van, centre, and rear as 12, 13, and 11 ships. Other sources regularize the force as twelve ships in each division. This reconstruction retains the uneven figures from its main source because the distribution can also fit the battle plan. The exchange shows that an order of battle is an interpretation assembled from sources rather than a neutral list. Apparent asymmetry may reflect tactical roles, documentary disagreement, or both.