Microsoft's share of shipped devices will climb slightly this year and pick up some steam in 2016, but Apple's share will grow at a more sluggish pace because of slow-downs in iPhone and iPad, Gartner forecast Monday.

For 2015, Windows' share of the operating systems on all devices -- smartphones, tablets, PCs, ultra-lights and hybrids -- will climb to 14.4%, up from 14% last year, Gartner said in new estimates. It claimed that shipments would increase by less than 7%, to 355 million.

As it did several times last year, Gartner downgraded Windows' numbers for 2015 Monday: Its October 2014 forecast pegged Windows at 14.6% by the end of this year.

Gartner projected Windows' share in 2016 would climb to 15.3% on the back of 396.3 million devices shipped, a year-over-year increase of almost 11%, the largest boost since 2013, when PC sales began a prolonged contraction.

Microsoft wasn't the only OS maker whose forecast worsened in Gartner's latest estimate. Apple will also grow its share at a slower tempo than anticipated by several predictions of 2014.

Apple finished 2014 with an operating system share of 11% by virtue of about 262.6 million devices shipped, said Gartner, and should see its slice of the OS pie grow to 11.3% in 2015. That's less than the 11.6% pegged in the October forecast.

The Cupertino, Calif. company's share will reach that 11.6% -- but now not until the end of 2016, Gartner said today.

Those numbers were significantly under the aggressive estimate Gartner touted a year ago; in January 2014, it predicted Apple's share would reach 13.9% in 2014 and a whopping 15.9% in 2016, hot on the heels of Windows.

They also represented year-over-year increases in devices shipped of 6% for this year and 7% for the next, the numbers in marked contrast to the double-digit growth Apple experienced in 2013 and 2014.

What happened to make Gartner change its prognostication tune?

Its analysts cited Apple's two largest-volume lines, the iPad and iPhone, for their change of heart, pointing -- like many other analysts have in 2014 -- to a longer-than-anticipated refresh cycle for Apple's tablet and the belief that Apple will find it tougher showing iPhone growth in the future as it runs low of new markets and has a difficult time topping the iPhone 6 line.

"The challenge for the next iPhone to find significant growth becomes greater [in 2015 and 2016]," Ranjit Atwal, a Gartner analyst, said in a statement.

Meanwhile, Gartner's forecast for Android got more bullish than ever on Monday. By the end of 2015, Android will have captured 58.9% of the device share -- up from an October 2014 forecast of 57.4% and a January 2014 bet of just 47.8% -- and will grow even larger in 2016, accounting for 62.9% of all smartphones and tablets in two years.