Netflix (NFLX - Get Report) may have hit a home run last quarter, but can it stay on the upswing?

The streaming giant's stock is up more than 5% since it released last quarter's earnings results on Tuesday, reporting that it blew away expectations on new subscribers, adding 6.96 million new subs versus the mere 5 million it had guided for previously.

It also forecast that it will add 9.4 million subscribers next quarter -- a normally strong quarter owing to the holidays -- which is 23% higher than Wall Street's consensus. 

Interest rates, content spend and average selling prices are just a few of the other issues that come up around Netflix earnings. But for investors, subscriber growth is an obsessively-watched metric and one that -- at least after Netflix's July earnings report -- produced such a strong reaction that it sent shares into a weeks-long slump from which it still hasn't fully recovered.

So does Netflix's rebound on subscribers last quarter mean that the second quarter's subscriber miss was merely a blip? Not necessarily -- it may be more a reflection of just how challenging it is for Netflix to accurately forecast subscriber growth.

"I was encouraged that they rebounded, but not terribly surprised that they did," said Moody's analyst Neil Begley. "They have a hard time predicting how many subscribers they're going to add."

There are several factors that make it challenging to forecast subscriber growth, relative to other business models: One is that Netflix is still new to many markets across the world, according to Begley: "In many of these countries they launched only a year and a half ago, and a lot of the success is in how aggressively they market, and word-of-mouth," he adds.

Layer on top of that the fact that they offer free trials -- and the idiosyncrasies in delivering content to different international markets, like Japan or India -- and it's no wonder that subscriber estimates versus reality sometimes don't line up. Netflix executives said on Tuesday that they'll stop reporting trial subscriptions and instead just focus on paid subscribers.

Another factor is that word of mouth -- the "Game of Thrones" effect that drives sign-ups on the strength of a single, wildly popular show -- varies by season by season. 

Many analysts purported that the Netflix's third-quarter content slate was better than the previous quarter's: "The content slate that came online during 3Q was better than 2Q's, including new seasons of big-name series such as Orange is the New Black, Ozark, and BoJack Horseman," said Nomura's Mark Kelley earlier this week.

But it's not always easy to forecast what shows will wind up with so much buzz, either on social media around the water cooler, that they compel people to shell out en masse to see it. For Netflix investors, that unpredictability may just be part of the game. 

"I think that's just the nature of business," Begley added.