Why the difference?
I'm not sure but I think this is a handicap of painting with oils. Or maybe it's the handicap of being impatient. :)
The difference was my appaloosa foal and pinto horse mini paintings were done all at once, wet-on-wet, because I tend to be impatient about waiting days before I can continue working on a particular painting, especially if they are small. I know hours-wise, I can get a small painting like this done within a day ... if I work wet-on-wet. If I wait for things to dry between layers it can take days. Taking several days to complete such a small painting is something I find hard to reconcile, so I force it to be done in a day. This requires wet-on-wet painting when you work with oils. There's just no way around it.
Now, there are many cases where wet-on-wet is a good thing, something artists intentionally do to achieve a specific effect. In fact, this works fine for me on my larger paintings because I like the blending I can do on fine detail with wet-on-wet. But for my mini paintings, I think I'm starting to like a wet-on-dry look better. Because such small paintings do not allow for fine detail, I think the effect of having distinctive layers adds more to the painting. The wet-on-wet just isn't as effective on such small works. So, because I was recently working on several small paintings at once, I was sort of forced to wait between layers to work on one of these small paintings. I've discovered it is more efficient and effective that way. That's not to say the wet-on-wet mini paintings are bad. They have their own unique qualities that some people may actually find more appealing. In fact, this wet-on-wet is my own personal preference for larger works but wet-on-dry works better for smaller works for me. Others may or may not agree. What do you think?