Identities are not formed overnight. People change and evolve constantly. Think about it, do you have the exact same personality now as you did ten or fifteen years ago? (If you answered "yes," you might consider getting out more.) Musicians are, obviously, real human individuals with ever-changing characteristics, but we somehow expect the music they create today to sound nearly the same as it did a decade ago. And that's just not realistic.
Besides, in our heart of hearts, do we really want them to keep making variations of the same song for their entire careers? Do we really need another Nickelback? Still, that doesn't mean we can't be surprised at just how drastically a musician has altered their sonic style. After all, who could have imagined that simple reggae group was once a hardcore thrash metal band?
So, just as your wild and crazy college roommate eventually stops doing beer bongs and starts preparing his taxes on schedule every year, a band in their formative years are going to sound a lot different than they will once they've matured.
Which makes it a lot of fun (or sometimes just a little depressing) to go back and compare their current sound to the one they had when no one knew who they were.