Image source: Sirius XM.

The number of people betting against Sirius XM Radio (SIRI 0.36%) continues to shrink. There were just 145.5 million shares of the satellite radio provider sold short as of mid-January. Yes, that's a lot, but it's actually the lowest number of shares shorted since mid-July.

Short interest may be at its lowest level in six months, but naysayers placing buy orders to close out their bearish positions haven't exactly given the stock a boost. The stock ended last week at $3.42, 19% below the multiyear high it hit three months ago. It's also dangerously close to revisiting its 52-week low of $3.31.

That's actually a pretty tight trading range for Sirius XM. There's just a 21% gap between its 52-week high and its 52-week low, a historical anomaly for a stock with a volatile past. Sirius XM's one-year beta -- the most popular market metric on volatility -- is now at a pedestrian 0.89. The steady share price is probably playing a starring role in the reduction of Sirius XM's short interest. The lack of volatility cuts both ways, leaving bullish and bearish speculators moving on to stocks with wilder price swings. 

The lack of interest on both ends of the enthusiasm spectrum comes at a time when Sirius XM is running on all cylinders. It just closed out the fourth quarter with 29.6 million subscribers, 2.3 million more than it had a year earlier. Conversion rates are running at a historic low -- it just dipped below 40% of buyers of new cars with factory-installed receivers for the first time -- but clearly Sirius XM is more than making up the difference in volume.

It sees growth continuing to move in the right direction. Its forecast calls for 31 million subscribers by the end of the year. Sirius XM's targets for 2016 of $4.9 billion in revenue, $1.78 billion in adjusted EBITDA, and $1.4 billion in free cash flow suggest growth of 7%, 7%, and 6%, respectively. That's well below its growth rates in the past, but at least it's moving in the right direction. 

Perhaps more importantly, Sirius XM has a habit of bumping its guidance higher as the year plays out. It was originally targeting just 1.2 million net additions in subscribers for 2015 at this point last year. The goal kept moving higher with every passing quarter, leaving it with 2.3 million net additions by year's end. 

It's odd to see buy and short interest dry up at the same time, but this doesn't mean that Sirius XM has gotten boring. There's more to Sirius XM than its low beta and growth targets suggest. Given Sirius XM's past and its huge gains since bottoming out seven years ago, even if it is boring, it won't stay that way for long.