No matter how you put the samples they always sounds awesome.
That's because the base harmony is always the same for every sound you choose. It's like they grabbed the source from some music, separated each sound and gave the user the choice to remix it like they want to. Since all the samples are from the same music, they will always fit together.
Obviously it isn't as straight forward as I'm putting it, since in the website the base harmony is always the same. On most tracks this base harmony varies conform the part of the music playing (verses, choruses, bridges, etc...)