The system is not "messed up", but basically your understanding of what manually installed means is incorrect. A package is never actually marked manual or not-manual. It can only ever be marked auto or not-auto.
A package is marked auto when it is installed automatically to satisfy a dependency. This means that it can (and implies that it should) be removed automatically when no other package depends on it any longer.
On a normal desktop system the majority of packages are installed when the OS is first installed by the installer. These packages have to be installed not-auto, or else they would be removed because nothing depends on them.
Take for example 'abiword' near the top of your list. This is a word processor that is probably quite normal to have installed on a desktop system. I assume that you used one of many flavours of desktop installers. It probably didn't ask you specifically if you wanted abiword. You probably didn't install abiword yourself by name. You still chose to have it by choosing the install medium that you used.
Since there is no other package on your system that depends on you having abiword installed in order to work, it cannot be marked auto or else it would be removed. This means it is not marked auto. apt-mark showmanual
lists the packages that are not marked auto.
Note also that many installers install everything non-auto even if they could mark them auto because something depends on them. Even the important core packages that hundreds of things depend on get marked non-auto. In this sense, maybe the installer is "messsed up", but it is nothing special about your system.
My best attempt at getting a list of things I might have installed myself is:
aptitude search -F '%p' --disable-columns '~i !~M !~E !~prequired !~pimportant !~pstandard !~tminimal !~tstandard'
To exclude packages that are depended on, use:
aptitude search -F '%p' --disable-columns '~i !~M !~R~i !~E !~prequired !~pimportant !~pstandard !~tminimal !~tstandard'