Actually, the adoption rate of new versions, not the growth rate, is the the important metric.

http://www.netmagazine.com/features/developers-guide-browser-adoption-rates

Notice how abnormal IE's adoption rates are? That's because IE is tied to OS version which means that developers need to remember 10-15 years worth of hacks to keep all of the various "in flight" versions of IE working.  Developers supporting browsers with faster adoption rates might need to remember only one year's worth of hacks.