[plt-scheme] Why .dmg for Mac source distributions?

I have downloaded and installed MzScheme from the Mac source code
distribution. It really struck me as uncommon that a source is
distributed in a dmg file. I've been using OS X and installing various
software for quite a while and haven't encountered a dmg being used
for this purpose before. A tarball is usual and the same tarball is
used for *NIX.

A tarball is certainly easier to deal with from the OS X command line.
For the disk image, just getting to the source code requires mounting
an unmounting the image which causes Finder to flip around a bit.

$ curl -O http://download.plt-scheme.org/bundles/4.2.1/mz/mz-4.2.1-src-mac.dmg
$ open mz-4.2.1-src-mac.dmg
$ cp -R /Volumes/MzScheme\ v4.2.1\ Source/MzScheme\ v4.2.1\ Source/
mz-4.2.1
$ umount /Volumes/MzScheme\ v4.2.1\ Source/
$ cd mz-4.2.1/src
$ ./configure --enable-xonx --prefix=/usr/local
...

verses the more straightforward and much more common style of a
tarball

$ curl -O http://download.plt-scheme.org/bundles/4.2.1/mz/mz-4.2.1-src-unix.tgz
$ tar xvzf mz-4.2.1-src-unix.tgz
$ cd mz-4.2.1/src
$ ./configure --enable-xonx --prefix=/usr/local
...

If there is no particular reason to be using a disk image, would a
switch to a tarball be better for the simple fact that it is more
common.

Peter