Since the first quarter of every year is known as the “slow season,” it isn't so hard to imagine that tablets, like everything else, would sell less.
The drop in shipments might still come across as a shock though. Over 20% isn't exactly a small percentage.
Indeed, shipments are expected to reach 33.8 million, give or take, for the January-March period.
While that still represents an improvement of 75.8% on-year, it is a 21.8% drop compared to the October-December period of 2012.
This will happen due to a decline of 46.9% in Android and Windows slates, and in spite of a rise in Apple iPad shipments (5%).
NVIDIA's Tegra 4, once it enters mass production, may or may not compensate for this drop later in the year.
Tablet Shipments to Fall 20% This Quarter
They are still much higher year-over-year though, not that it's a surprise
... so hot right now