Image source: Sirius XM.

There's some serious growth taking place at Sirius XM Radio (NASDAQ:SIRI), but the satellite radio provider is once again being too modest for its own good. Sirius XM closed out the year with 29.6 million subscribers by the end of 2015. That's 2.3 million more than it had at the end of 2014, and 0.6 million more than it had on its rolls at the end of the third quarter. That's important because in late October Sirius XM was forecasting that it would close out the year with just 29.3 million subs. Self-pay net additions also clocked in ahead of Sirius XM's outlook. 

One more positive nugget: Sirius XM now expects to meet or exceed its earlier guidance for revenue, adjusted EBITDA, and free cash flow for all of 2015. 

That may seem like a barrel of good news, but then Sirius XM followed that up with its initial guidance for 2016. That is where the fast-growing media giant fell short. It is targeting $4.9 billion in revenue, just shy of Wall Street's average estimate of $4.92 billion. That may not seem like much of a miss, but when you consider that Sirius XM is kicking off the year with 300,000 more subscribers than everyone thought it would, it does make it seem like a bigger miss.

We also have Sirius XM offering guidance of 1.4 million net subscriber additions in 2016. That would take the satellite radio monopoly to 31 million total subs by the end of next year, but you don't need to be a math geek to know that 1.4 million additions this year is a far cry from 2.3 million additions last year.

The silver lining here is that Sirius XM is a serial lowballer. If you think gaining 1.4 million more accounts this year is disappointing, you should've seen it at this point last year. Sirius XM was only eyeing 1.2 million net additions when it initiated its guidance for 2015. That tally rose to 1.4 million in April, 1.8 million in July, and 2 million in October. We now know that it wrapped up 2015 with 2.3 million net additions, nearly twice as many as it was originally expecting. 

There is no reason why investors shouldn't expect more of the same through 2016. Outside of auto sales stalling -- a big deal since that's the lifeblood of subscriber growth -- or some disruptive in-car technology emerging, Sirius XM should be able to keep bumping its targets higher with every passing quarter.

And, no, the connected car isn't that disruptive technology. Sirius XM has been living in a world of free or nearly free streaming apps for years, and it's still finding ways to penetrate the market. In fact, the only reason to worry here is if guidance actually remains intact as 2016 plays out. Modesty has served Sirius XM shareholders well, with the stock being one of the market's biggest winners since bottoming out in 2009. It would be a shame to lose that trait.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.