Q:

Differences between man pages

I often find myself reading man pages online instead of from the command line on my system.  Part of the reason is because I  find it easier to browse this information on a browser on the Internet than on the command line.  
However, this also means that I often end up reading information from "generic" man pages (e.g. from http://linuxmanpages.com/), or from a different distribution or tool version, and not from the actual specific tool/version I have on my system.  
So I always wondered if I am missing something by not reading the man pages installed on my machine.  I presume that for the most-commonly used and stable tools (and parameters), the differences are negligible, but I would like to know to what extent this is a safe assumption. 
More specifically:

Is it common for different distributions to come with different man pages?  Is this true even for the same version of the tools provided by the distro?  
What man pages can vary most significantly across distributions? 

A:

The man page for a given piece of software is usually written by its author(s).  The ones on your system are going to be the most appropriate for the tools you have installed, because they will have the correct information for your implementation and version level. 
Mostly, the differences are insignificant, but the biggest difference is between GNU, BSD, and BusyBox tools. 

A:

There are a few programs that have several different implementations, which are compatible as far as their main features are concerned but differ in details or extensions and possibly have documentation that was written independently.  When that happens, you'll see different man pages on different systems, since these are technically different programs. 
A few examples on Linux are cron (there are several implementations around), init and friends (there are a lot of differences between SysVinit, Upstart and Systemd), awk (most Linux systems have GNU awk but there is also the smaller Mawk), locate (there's GNU locate, and then there are mlocate and slocate), …
Additionally, some programs have optional features that are selected by compilation options.  Often the man page is generated at compile time and only documents the options included in that particular compilation. 
Some distributions put up their manual pages online (this can be useful to check portability), for example Debian, SuSE, Ubuntu, … The links should be on the distribution's tag wiki on Unix Stack Exchange (if a distribution has man pages online and the link isn't in the wiki, please add it).