Nonaligned lines...is it normal?

To align lines on different pages (or columns) you need to typeset "on a grid". (https://en.wikipedia.org/wiki/Grid_%28graphic_design%29). That's rather difficult in LaTeX. There are a lot of irregular spaces and line heights which don't fit on a grid, e.g. sectioning commands, display math, lists.

But in your case where only normal text is on the page you are getting the misaligned lines because your page layout is wrong: With \leading{16pt} you are changing the line spacing and the textheight is no longer a multiple of the leading and so LaTeX uses the stretchable \parskip to flush align the lines at the bottom.

Move the \leading{16pt} before the \checkandfixlayout command and you will get perfectly aligned pages. Setting the \parskip to 0pt is not a sensible solution, it will only lead to lots of warnings like this:

Underfull \vbox (badness 10000) has occurred while \output is active []

Btw: In a high quality book you should never use \sloppy globally. That should be only a last resort for problematic paragraphs.

This is dependent on the length within \parskip. If it has some glue, it may stretch to better-fit the page. In your case, I see

> 0.0pt plus 1.0pt
> l.22 \showthe\parskip

implying that your \parskip could be anything from 0pt to 1pt, which leads to what you see as a mis-alignment within the baselines.

Is this normal? Sure. It allows for some flexibility with the page construction. Otherwise, non-baseline-constructions (like figures) could cause issues on the page.

Can it be changed? Yes. Simply issuing \setlength{\parskip}{1\parskip} removes the glue. Sometimes people would issue \raggedbottom to avoid underfilled pages if there is no glue available to stretch content.