Problems removing orphan packages

[方坤]

Last week, I ran `cd /usr/ports/devel/git ; make install clean` to install
the Git package,
I found that the "/deve/bison", "/shells/bash" and "www/w3m" packages also
installed
as the "devel/git" package need them.

Today I ran `portmaster -s` to uninstall unused packages, I found the
"/deve/bison",
"/shells/bash" and "www/w3m" packages were on the listed. So I uninstalled
all the
packages in that list.

I didn't understand why portmaster thinks that the "/deve/bison",
"/shells/bash" and
"www/w3m" packages were unused packages. So I ran
`cd /usr/ports/devel/git ; make install clean` again, I found that the
"/deve/bison",
"/shells/bash" and "www/w3m" packages installed again.

Is anyone can teach me why?