The tablet market is 100 million units smaller than expected. What happened?

This past week included two milestones for the tablet market: It was the first full week of sales for Apple’s newest iPad Pro, and it was also the sixth anniversary of the first iPad’s debut in 2010.

In that time, Apple has cumulatively sold more than 300 million iPads. And while that’s an impressive number, the tablet market that Apple more or less created hasn’t met its expectations.

Early predictions for tablet sales assumed nothing less than a revolution in mobile computing. In 2011, as iPad sales accelerated and a broader market began to take shape, analysts at the research firm Gartner projected that annual tablet shipments would pass 300 million units by 2015, including almost 150 million iPads.

Reality has fallen well short of those forecasts. In February, research firm IDC estimated 2015’s tablet shipment total at 207 million units. (Apple sold 50 million iPads in 2015.)

Another data point: In January, Gartner’s revised estimates pegged the market for what it now calls “basic and utility ultramobiles” — a category that includes the iPad, Samsung Galaxy Tab and Nexus 7, among others– at 196 million shipments in 2015, and expects it to remain flat through 2018.

Either way you look at it, the market that materialized in 2015 fell short of what was expected five years ago by about 100 million units.

What happened? The smartphone, for one thing. The world’s most ubiquitous Internet-enabled device continues its reign, clocking sales of nearly 1.4 billion devices last year.

Meanwhile, the size of the displays on those smartphones has increased significantly over time to 4.7 inches as of last year, obviating the need for a tablet in many cases. If the North American market is any guide, larger screen sizes of five inches and above account for 70 percent of the market, according to the research firm GfK, and more than half of the market in China.

And the tablets people bought seem to have remained in active use for longer. Apple CEO Tim Cook said as much during a conference call a year ago. Consumers upgrade their tablets less often than their phones, but maybe more often than their PCs.

I fit that demographic exactly: I bought an iPad Air in late 2013 that I still use daily and don’t anticipate an upgrade this year. I will, however, probably upgrade my iPhone. I bought my last Mac in 2011.

Even so, there is another force in mobile computing brewing, though it’s smaller, quieter and coming from a less-expected place. Tablet sales overall have been declining — down 10 percent year over year in 2015. But sales of what IDC calls “detachables” — tablets that optionally attach to a keyboard of some kind, to somewhat convincingly mimic a laptop — have have been growing like crazy. (IDC considers the iPad Pro in this class.) One of every five tablets sold in Europe in the fourth quarter was a detachable, the firm says.

Their dual appeal can’t help but eat into two market segments: Conventional standalone tablets like the “classic” iPad, and of the old-school notebook PC, which has been on a long, slow decline of its own for the last several years.

Detachables are proving so popular that IDC reckons by 2020 they’ll account for about one-fifth of the entire market for what it calls “client computing devices,” which includes both tablets and PCs. It may not amount to the radical revolution that the overly eager analysts of 2011 had called for, but it will do.

This article originally appeared on Recode.net.