There may be more than one version of the same command on your system.
If there is more than one version of a program online, whereis tells you about all the versions that it finds. For example, on our system we have two versions of ls:
% whereis ls ls: /usr/5bin/ls /usr/bin/ls
The reason there are two versions of the same command has to do with the evolution of UNIX. The ls command developed differently on different versions of UNIX. Today, some modern operating systems provide both versions, so users can choose the one they’re used to. On our system, the System V versions of programs are stored in /usr/5bin.