I’d assume because you’re actively listening to music that you like and pick out. Maybe you’re singing along to it, or you’re actually listening to and interpreting the lyrics, tapping along to the sounds, you’re aware of every song that comes on. With radio music, it’s easy to passively listen to it, you’re not really fully aware of every song that comes on, maybe every now and then something plays that you recognize/like and you sing along to that, but then there’s probably a lot that you just tune out and ignore. It’s easier to “listen” to longer stretches of music if it’s just sort of background sounds you’re not as focused on.