Why does new file created by touch have GID wheel?

/var/tmp has the sticky bit set:

pse@Mithos:~$ ll -d /var/tmp
drwxrwxrwt  5 root  wheel  170 Dec 24 16:17 /var/tmp/

so any files created in there inherit the group of the directory.

From man sticky:

A directory whose `sticky bit' is set becomes an append-only directory, or, more accurately, a directory in which the deletion of files is restricted. A file in a sticky directory may only be removed or renamed by a user if the user has write permission for the directory and the user is the owner of the file, the owner of the directory, or the super-user. This feature is usefully applied to directories such as /tmp which must be publicly writable but should deny users the license to arbitrarily delete or rename each others' files.

Any user may create a sticky directory. See chmod(1) for details about modifying file modes.