Included in Android's design guidelines is a section regarding iconography. The guidelines give very specific instructions on how to design a launcher icon for Android - it should have a unique silhouette, it should have a slight downward perspective, and it should be clearly visible no matter what wallpaper is behind it.
Many have opined, however, that it's odd that Google maintains different iconography for its apps on Android and their corresponding web services. For instance, on the web, Gmail's icon is a flat, closed envelope, with a long shadow, while on Android it's a more dimensional open envelope with the M appearing on the "paper" inside.
Gmail's web icon vs Android counterpart
It could be argued that having different icons for different platforms makes sense, but if today's rumor is to be believed, Android's icons may soon take a step toward the style of their web counterparts.
Source : http://www.androidpolice.com/2014/04/14/rumor-googles-android-app-icons-to-get-a-moonshine-makeover/