aptitude: packages have been kept back?
I'm a long time apt-get user, trying to make sense of aptitude.
I really like aptitude's automatic dependancy tracking; but it keeps
"keeping packages back" whenever I run "aptitude dist-upgrade".
"apt-get dist-upgrade" doesn't keep packages back.
Examining these "kept back" packages in aptitude, they are marked "id",
which I gather means "installed" and "will be deleted". I can think of
no reason why these packages should be "kept back" or "deleted", nor
have I been able to find such a reason using aptitude.
Can someone please explain why some packages might be "kept back", or
how I might find out?