Netflix Inc (NFLX -0.79%) posted another blowout quarter earlier this week, but this one was even better than its average subscriber beat.

Netflix's second-quarter subscriber growth crushed its own forecast of 3.2 million, as the global streaming service signed up 5.2 million new members in the period. It was a record for the second quarter of the year, as the summer months are usually slow for Netflix, and the company even bucked normal seasonality trends by posting sequential growth over the first quarter. For comparison, it added just 1.7 million new members in the quarter a year ago. 

Netflix's LA offices

Image source: Netflix.

Sign-ups were strong both at home and abroad: Netflix added 1.07 million new members in the U.S., a new record for the quarter, and 4.14 million internationally, also its best ever for the second quarter and the third-best total for any quarter.

It's all about content

There's only one possible explanation for Netflix's explosive growth last quarter: content. Without any new territories to expand to, Netflix must attract new subscribers with its entertainment, and it did so last quarter with its biggest-ever slate of original releases.

Those include 14 new seasons of series, 13 comedy specials, six documentaries, two documentary series, nine feature films, and seven seasons of TV for kids. That level of production for just a three-month period puts Netflix in a league of its own. Not one of competitors -- including HBO, network TV stations, or Amazon -- can compare with that output.

CEO Reed Hastings put all the credit for the quarterly growth on the content, saying on the conference call, "I think we're just seeing the rewards of doing great content." The quarter's releases included popular debut shows like 13 Reasons Why and GLOW, and new seasons of the two series that have become the company's tentpoles, House of Cards and Orange is the New Black. The film Okja also received a standing ovation at the Cannes Film Festival and could be the company's first feature film hit.

Not just quantity

The breadth of Netflix's content may explain its accelerating subscriber growth, but it's not just the quantity that's improving. It's also the quality. The streamer received a company record 91 Emmy nominations when they were announced last week. As the chart below shows, nominations have been soaring as the company has dived into original content. 

Netflix has gone from just 13 Emmy nominations in 2013 to 91 this year, challenging Time Warner's (TWX) HBO for supremacy. Five of the 14 series that were nominated for best drama or comedy went to Netflix shows, including Stranger ThingsThe Crown, House of Cards, Master of None, and Unbreakable Kimmy Schmidt. HBO scored 111 nominations to lead the pack, but Netflix's surge begs the question of when it will overtake HBO as the critical darling for television programming. Hastings would certainly claim it as a big prize if his company beat HBO at the Emmy Awards.

The lesson here for Netflix investors is that the growing investments in content are overwhelmingly justified. While some observers, like my colleague Evan Niu, worry about the increasing debt load, the acceleration in subscriber growth shows that it's worth it. Netflix grew from 50 million to 100 million subscribers in just three years. With a growing and improving slate of content that's made the company the first name in home entertainment, Netflix could hit 150 million even faster than that.