The original Pixel may have contributed significantly to this figure as well and to the claim of unit shipments doubling than the previous year because the original Pixel was launched in October 2016 and it continued to sell well into 2017.
The IDC says Google shipped only 3.9 million Pixel phones in 2017. The Pixel phones are excellent, but Google just doesn't have much success in selling them. The numbers suggest that Google still has a long way to go before it can challenge the iPhones. That's including both the original, first-generation Pixel handsets and the Pixel 2 second-generation which launched later in the year.More news: South Korean president to meet North Korean leader's sister
The Pixel range debuted in 2016 and got a refresh late past year, which brought us the Pixel 2 and Pixel 2 XL, so it's a relatively new brand when compared to iPhone and Galaxy S. Its availability is also limited to a handful of markets. That's not a massive number, but it shows great growth for the company's lineup.
The third reason is that Google doesn't have a strong retail presence. Google sells Pixel phones only through Verizon and its own website, leaving out AT&T, T-Mobile, and Sprint. Google's cheapest phone is last year's Pixel 1, which got a $100 price drop when the Pixel 2 came out and is technically for sale for $549 (although it's still sold out). Apple also has deals with all the major carriers in the U.S., putting iPhones in even more retail locations with even more salespeople.
Solving the stock issues with the Pixel 2 this year is a big improvement, but if Google really wants to compete in sales numbers, it has many more problems to solve.