Because they're close enough to each other to be considered one language. But there are differences between the two languages. But it's small, kinda like British and American (but moreso). Lots of things are spelled and pronounced differenty.
Ah, one language but some spelling differences and things... that makes sense. But it still is one language, really... just like British, American, Australian, and all the other varieties of English are all English. :)
*and in a big DUH moment remembers that he knows Machek is male cat and Machka (with hte c with hat, not ch, of course) is female cat... given that our cat's name is Machek...* :D