there are preferences files, to set priorities for some branches like testing or sid,
and if you want to "overrule" your prefs then you need
apt-get -t unstable pkg_name
this is for example usefull, if you have a testing system, and want some pkg from unstable, but not all of them.
this way you can have both in your sources.list...without having to worry about what happens when you do "apt-get dist-upgrade"
but if you dont have those preferences files(default) then you will always get the highest vers numbers from the packages, which are most often in sid. and a apt-get dist-upgrade, will always end up in unstable...
so it depends on your setup