testing & stable: vanished packages?

[furufuru@ccsr.u-tokyo.ac.jp]

Hi all,

I'm wondering why some packages aren't available for
the testing distribution.  For example, I wanted to
install apcalc (in the math section) but learned that
it's available only for the unstable and stable
distributions, not for the testing. (I searched
at http://www.debian.org .)

When sarge became the stable distribution, I read
that initially the stable and testing distributions
are the same but that packages in the testing keep
upgraded, if I remember correctly.  If that's so,
some packages in the testing will get frequently
upgraded and others will get upgraded less frequent
or not at all, but there's no reason why a package
must be removed, is there?  (unless there's a security
problem or some such serious problem.)

I downloaded .deb files in the stable distribution
of apcalc and installed them using "dpkg -i".
I'm not very unhappy with that.  But, I remain
puzzled.

Cheers,
Ryo