We already know one reason Google's earnings came in light—Motorola was a bigger mess than expected.

Here's another reason, and one that's not as easily fixed as Motorola. (Not that Motorola is an easy fix.)

In its most recent quarter, stock-based compensation jumped 33 percent to $762 million from $571 million in the same quarter a year ago. That differential accounted for a good chunk of Google's earnings miss. ($47 million of that was due to a restructuring charge.) 

For all of 2012, Google expects to record $2.7 billion in charges for stock-based compensation.

We've heard about Google making big offers to keep key personnel from defecting—especially to a short list of hated competitors in Silicon Valley like Facebook, Twitter, and Square.

Besides keeping employees from jumping to rivals, Google has to worry about startups luring engineers and product designers away with the promise of a big payday.

So it's been lavishing big stock awards on key personnel, new and old.

One by one, an eight-figure package may not seem like much when Google's taking in $14 billion a quarter. But $10 million here, $20 million there starts to add up.