Just as I go through phases liking curvy ladies to slightly slimmer Oriental ladies to a leather clad dominatrix to a "girl next door" (with a kinky side), etc., I seem to vary the length of my captions. Sometimes I like a story, other times the caption is like a single panel from a comic.
The long caption is like I set the scene, and explore the fantasy in depth. The short caption is like popcorn, or just exploring a single moment.
As with any of this, I have no idea why. Just enjoy...