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?
Idk
Depends on the hardness of the problems I think. Also, I think the last 2 problems are a lot harder in div1+div2 rounds than in Div 1 only rounds (in general). So it depends on how the co-ordinators gauge the difficulty of the problems proposed by the round authors
It doesn't always depend on the difficulty of the problems. Sometimes it can be requested to make the round combined division (in my experience, usually when prizes are involved) by contest authors.
The split is made mainly to let Div. 1 participants solve harder problems rather than speedtyping easy Div. 2 AB problems
Combined rounds are usually created to invite more people to join with prizes, merchandise etc. One might notice such rounds usually take longer than 2 hours.