Though, the first clause means that all animals are equal, however it does not expresses this claim openly. Therefore, it is like misreading this word “equal” as a relative term than the absolute one, which means equal-ness has different degrees. It refers to some form of corruption happening on Animal farm, which is also the theme of this novel.
The initial phrase “all animals” refers to whole world of animals with equality. The revised phrase “some animals” specifies the uniqueness, elite nature and chosen status of the “some.” The pigs envision themselves as “some” privileged, and under their totalitarian command, the working class of animals only exist to serve their glory of leadership, provide food, comfort and support their exclusive and luxurious lifestyle.