To reproduce the effect:

1. create a new Impress presentation
2. select the normal "Title + bulletd list" style for a slide
3. enter three or four lines of text, each as a new (bulleted) item
4. select one character in the second line and 
   change its text style to subscript
5. observation: the line spacing between the second and third line increases

This behavior is unwanted, I think. It screws up the visual appearance of any
itemized list, where you occasionally have some subscript characters in.

The behavior of both OO-Writer 3.1 as well as Powerpoint is different and more
useful: There the line spacing does not change.