Macs aren't actually linux, they are based (a port I think) on Unix though and are very close relatives to linux.
There is a distinction because you will notice much larger differences between Mac and [insert linux distro here] compared to linux distros to each other.
They are not "very close relatives to linux" unless you also think Solaris, Windows+cygwin, FreeBSD etc. are also very close relatives to Linux. In which case the term becomes meaningless because almost every mainstream OS is a close relative of every other one.
There is a distinction because you will notice much larger differences between Mac and [insert linux distro here] compared to linux distros to each other.