If the entire clothing rack outside the hotel suite doesn't do the trick, the entourage tells us that we are in the presence of a major celebrity.

Ashlee Simpson travels in a pack, with hairstylist, makeup artist and assorted other assistants in tow. They are with her as she breezes into the suite, giggling with her as she spends an extended period in the adjoining bedroom trying to find the right pair of pants and piling on the compliments as she poses for photos. They're all close to her age, they're all friends, and their world - on this day, and probably many more - clearly revolves around the 21-year-old currently topping Billboard's album chart.

When the interview starts, though, her entourage respectfully file out. And it's when she's alone that one realizes that despite her fame and growing fortune, the most remarkable thing about Simpson is how utterly unremarkable she is.

Sipping on a Red Bull and offering confident, polite and possibly well-rehearsed answers, she is neither especially witty nor as famously ditzy as her sister Jessica, the singing, tabloid-topping reality-TV star whose fame preceded hers. She is neither warm nor cold; neither unattractive nor unusually striking; neither as edgy as her early marketing sold her nor as sexy as the more recent efforts would suggest. She is just ... there.

The same, really, could be said for her chart-topping sophomore album, I Am Me, which in truth is not as awful as some of its reviews have suggested. Passable in a forgettable sort of way, it is the sort of thing an endless number of young, reasonably competent singers could accomplish in the hands of the right producer.

All this is no doubt the sort of assessment that prompts her to insist that she never reads her own press. ("I don't know if [critics] sit around and actually listen to the record as if they would be a fan of mine," she assesses, not incorrectly.) But Simpson's unremarkability is very likely the secret behind her success.

By this point, she could reasonably have been expected to be relegated to playing shopping malls, not just visiting them for autograph sessions as she did later yesterday. In the shadow of her older, blonder sister from the get-go, she briefly appeared slated to become the next Milli Vanilli after the most humiliating musical performance in Saturday Night Live's three-decade history. And yet here she is, mere months after being exposed on national television as a lip-syncer (and questionable jig-dancer), wildly outselling more seasoned artists.

Pressed to explain this phenomenon, her brief response at first seems woefully inadequate. "I think that I have a great fan base, and I think that they want to hear my music, and I feel that there's a huge group of people that can actually relate to it," she offers, then waits politely for the next question.

But there is more than a hint of truth to that last part. Every star geared toward "teens and 'tweens," as Simpson identifies her biggest supporters, will go on ad nauseum about how their fans relate to them. But much as they may try to emulate them, it's hard to imagine there are many 13-year-olds who can actually relate to sultry, oversexed stars like Britney Spears, Lindsay Lohan and, well, Jessica Simpson, nor to hyper-cute girl-next-door types like Hillary Duff.

Ashlee, on the other hand, is sufficiently unimposing that hundreds of thousands of girls can easily imagine being in her shoes - the sort of girl who grows up dreaming of showbiz (if she wasn't a singer, she suggests she'd probably be a makeup artist) and somehow beats the odds by actually making it.

Perhaps her manager/father, the slightly unsettling force behind both Simpson sisters, identified this selling point. Perhaps she just stumbled into it. But image-wise, it is certainly more important than her back-and-forth shifts from blonde to brunette or rock-chick to pop star.

She has no great interest in earning accolades ("I never make music to be critically acclaimed"), takes little umbrage at the perception that producers are behind her success ("sometimes they can take your idea to the next level"), and has no desire to break free from her family ("they're a great support system"). She is just happy to be ... there. And so long as that's all she is, she'll likely keep topping the charts.