Variations Among Unix Versions
There are a lot of different versions of Unix. Unix originated at AT&T corporation's Bell Labs about 40 years ago. It became a commercial product. AT&T licensed it to various other commercial organizations who then adapted and changed it in various ways to produce their own flavors of it. Today there are a number of commercial versions. IBM has one called AIX. Hewlett-Packard has HP-UX. Sun Microsystems had Solaris. These are being phased out in most installations that still retain them. There are also versions that are available for free and whose code base stems partly or wholly from sources other than the original AT&T code. Primary among them are FreeBSD, NetBSD, and OpenBSD. Descended from the former is Apple's OS X. And there is Linux, written from scratch rather than derived from the Unix code base but by design closely functionally equivalent.
The bulk of the behaviors and commands on any of the Unixes resembles that which is found on any of the others. But there are always differences too. These differences are not unusual. Generally, when you want the authoritative documentation for a command on a given system where you may be working, you can rely on the documentation printed for that command by the "man" command.
Please compare and contrast, noting the differences between the documentation for the "who" command from a FreeBSD system, versus that from a Linux system. In both cases the documentation was produced by issuing the command "man who." You can see that the commands and their options are not exactly the same.