Page not completely used

I realized in my document (book class, flushbottom by default) that not all pages are equally stretched until the bottom. At some points, where at the next page starts a new section, the text before ends not exactly at the bottom, more white space than it should be is there. Interesting, if I add one line, to stretch the content, rather the text moves to the next page.

Does anyone know where the problem might be? It looks weird.

documentclass[12pt,twoside, openany]{book}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc} % Output font encoding for international characters
usepackage[greek,english]{babel}
usepackage{indentfirst} % Identation after chapter header
usepackage{graphicx}
graphicspath{{images/} }
usepackage[labelfont={bf},
        %belowskip=-10pt,
           %aboveskip=0pt
        singlelinecheck=false
       ]{caption}                           
usepackage{romannum}
usepackage[toc,page]{appendix}
usepackage{tabu}
usepackage{xltabular}
usepackage{booktabs,tabularx}
usepackage{pgfplots}
usepackage{boldline}
usepackage{epstopdf}
usepackage{multirow}
usepackage{amsmath}
usepackage{bm}
usepackage{enumitem}
usepackage{float}
usepackage{wrapfig}
usepackage{subfigure}
usepackage{xcolor}
usepackage{array}
newcolumntype{P}[1]{>{centeringarraybackslash}p{#1}}
usepackage{comment}
usepackage{textgreek}
usepackage{parskip}
setlength{parskip}{baselineskip}

usepackage{url}
urlstyle{same}
usepackage[backend=biber,natbib=true,sorting=none,isbn=false,eprint=false,
doi=false,style=ieee,dashed=false,citestyle=numeric-comp]{biblatex}
AtEveryBibitem{clearfield{number}}
addbibresource{Reference.bib}
usepackage[intoc]{nomencl} % List of symbols
usepackage{makeidx}
makenomenclature
renewcommand{nomname}{List of Abbreviations}

usepackage{setspace}
usepackage{titlesec}
titlespacing*{section} % Spacing before/after (sub-) section titles
{0pt}{16pt plus 2pt minus 2pt}{18pt plus 2pt minus 2pt}
titlespacing*{subsection}
{0pt}{12pt plus 2pt minus 2pt}{10pt plus 2pt minus 2pt}
setcounter{secnumdepth}{4}
setcounter{tocdepth}{4}
titleformat{chapter}[hang]{Hugebfseries}{thechapter{. }}{0pt} 
{Hugebfseries} % Title without "Chapter" but Nr. in front

usepackage{amsmath}
usepackage{geometry}


usepackage{siunitx}
usepackage[acronym,toc]{glossaries}              % use glossaries-package


setlength{glsdescwidth}{15cm}

newglossary[slg]{symbolslist}{syi}{syg}{List of Symbols} % create add. symbolslist


glsaddkey{unit}{glsentrytext{glslabel}}{glsentryunit}{GLsentryunit}{glsunit}{Glsunit}{GLSunit}

makeglossaries                                   % activate glossaries-package


renewcommand*{glossaryheader}{%  Change the table header
%bfseries Sign & bfseries Description & bfseries Unit 
%hline
endhead}
renewcommand*{glossentry}[2]{%  Change the displayed items
glstarget{##1}{glossentryname{##1}} %
& glossentrydesc{##1}% Description
& glsunit{##1}  tabularnewline
 }
 }



geometry{
paper=a4paper, % Change to letterpaper for US letter
inner=2.5cm, % Inner margin
outer=2.5cm, % Outer margin
bindingoffset=.5cm, % Binding offset
top=1.5cm, % Top margin
bottom=1.5cm, % Bottom margin
headheight=15pt 
}

usepackage{fancyhdr}
pagestyle{fancy}
fancyhf{}
fancyhead[LE,RO]{}
fancyhead[RE,LO]{rightmark}
fancyfoot[RE,LO]{}
fancyfoot[CE,CO]{thepage}
renewcommand{headrulewidth}{0.4pt}
renewcommand{footrulewidth}{0.4pt}
%setlength{headsep}{0.4 cm} % Spacing between header and textblock below
setlength{footskip}{15pt}
usepackage[bottom]{footmisc}
fancyheadoffset{0pt}
fancyfootoffset{0pt}
%geometry{headheight=29.54742pt}
%setlength{headheight}{30pt}

%setlength{footnotesep}{0.5 cm}