The KOMA-Script guide states the following on page 26:
The literature gives different values for good line lengths, depending on the author. To some extent, this is related to the native language of the author. Since the eye jumps from word to word, short words make this task easier. Considering all languages and fonts, a line length of 60 to 70 characters, including spaces and punctuation, forms a usable compromise. This requires well-chosen leading, but LaTeX's default is usually good enough. Longer line lengths should only be considered for highly-developed readers who spend several hours daily reading. However, even for such readers, line lengths greater than 80 characters are unsuitable.
Yet, the KOMA-Script guide has a line length of > 100 characters. Why is that? Are there exceptions to these rules that apply here?
Similarly, the guide states the following on page 24, which is apparently also violated by the KOMA-Script guide:
In a single-sided document, the left and the right margin should have equal widths. The relation of the upper margin to the lower margin should be 1:2. In a double-sided document (e. g. a book) however, the complete inner margin (the margin at the spine) should be the same as each of the two outer margins; in other words, a single page contributes only half of the inner margin.
Are there any special reasons for the differences between the claims being made and the actual layout being used in the same document?
An answer I found here indicates that the layout of the book is sound while the online version deviates and, further, that "margins have been cut to allow more legible text on the typical computer screen. You can look upon this as a service for the user or as one of the restrictions of the free version compared to the book." Yet, why would longer lines be legible on a computer screen if the following statement from the KOMA-Script guide is true (as mentioned above): "Longer line lengths should only be considered for highly-developed readers who spend several hours daily reading. However, even for such readers, line lengths greater than 80 characters are unsuitable."