Answer by garyjohn for Where are these non-default Vim settings stored if not in my .vimrc file?

To see where a particular option was last set, execute

:verbose set <option>?

To see the options that vim sets by itself, start vim as

vim -N -u NONE

then execute :set. In my case, running Vim 7.3.487 on Linux, I see

:set--- Options ---  helplang=en         scroll=11           ttymouse=xterm2noloadplugins         ttyfast  fileencodings=ucs-bom,utf-8,default,latin1  background=light

To find out why any of those are set that way, use :help. For example, :help 'ttyfast' explains that it is set because vim determinded that my terminal is an xterm.