As you can see from Contests, the recent CF round (#844) was (Div. 1 + Div. 2). But there are some rounds (such as #829, or the one scheduled on Feb 5, 2023) which are held as two contests, one for Div. 1 and another for Div. 2.
What is the reason to split some contests into two divisions, and combine some other contests? Is it done for more variety in the type of contests, or based on specific features of the problemset, or something else?