Why is there a difference in the command lines used in the different distros? like the commands used in ubuntu are a little different in fedora, why is that? is it because of the difference in the source codes?

There are several different shells that are in use in various distros, and each user has a default shell that is used as well as a default settings for those shells. What specific differences are you asking about?

Generally most Linux distros have similar commands. Some distros create tools specifically for their distro, and this one reason why you may find certain commands in one distro, but not in another. In some cases different packages can provide similar tools (e.g. awk, gawk, mawk all provide the awk command), so commands may differ slightly depending on which packages a distro chooses to use.

...and, to add to the previous answer, with some utilities you have the option of the 'full fat' version, with all of the options, or a slimmed down version. Distros generally know their intended market and take the decision between the options in line with the wants of their target market.

There are some more fundamental differences as well; when a distro selects a package manger format, then they implicitly select to use utilities that work with that package manager format and reject the tools the only work with incompatible package formats.