[Tex/LaTex] Variability in the line spacing of paragraph breaks

compilingkoma-scriptline-spacing

Don't know how to describe my problem properly, but there seems to be a variablility in the line spacing of paragraph breaks, with my settings or generally or anything. Since I don't know what is effecting this, I copied just some lines of the preamble, which might have some relevance to the problem and two screenshots, where you can see, what I mean. 

\documentclass[paper=15cm:23cm, headinclude=true, draft]{scrbook}

\renewcommand{\footnotesize}{\fontsize{8.0pt}{9.5pt}\selectfont}
\renewcommand{\normalsize}{\fontsize{9.7pt}{10.5pt}\selectfont}

\areaset[10mm]{112mm}{194mm}

The spaces between the Paragraph are slightly different.. Why?
I also get the message: 

Package typearea Warning: Maybe no optimal type area settings! 
                          The maximum limit of line width is about 16%
(typearea)                larger than the heuristically detected line width.
(typearea)                You may e.g. increase DIV, decrease fontsize
(typearea)                or change the papersize.

I would like to get a consistent spacing, which seems for me more in the right way, than an inconsistent one. I also never saw this in books (academic publishing).

Example one
Example two

Best Answer

The distance between baselines within a paragraph is given by the length \baselineskip. So, in your definition of \normalsize, you use

\fontsize{9.7pt}{10.5pt}\selectfont

which sets \baselineskip=10.5pt. That is, the baselines in your paragraph will be 10.5pt apart. Note that this is a property of a paragraph, and not of a line (see Only one \baselineskip per paragraph).

The distance between paragraphs is further extended by the length \parskip, which contains glue. This glue allows the paragraphs to stretch further apart or shrink closer depending on the amount of content (given by 0.0pt plus 1.0pt in your case, allowing for 1pt of stretch). So, you may see that the space between paragraphs (given by \parskip) differs from one page to the next. This is perfectly normal to obtain an optimal layout.

Specific to the KOMA-script classes, the KOMA-script documentation (section 3.1.3 Options for Document Layout, p 45) mentions:

The standard classes normally set paragraphs indented and without any vertical inter-paragraph space. This is the best solution when using a regular page layout, like the ones produced with the typearea package. [...] A combination of indentation and vertical inter-paragraph spacing is redundant and therefore should be avoided. The indentation is perfectly sufficient by itself. The only drawback of indentation is the reduction of the line length. The use of inter-paragraph spacing is therefore justified when using short lines, for instance in a newspaper.

Related Question