The install count on iOS/mobile is definitely a faster growing segment due to a huge market that has yet to buy in. Proportionately, there are fewer working individuals in the 1st and 2nd world that haven't access to a desktop. The stats then are a reflection of a growth industry adding new consumers versus a mature industry that only sees replacements.
In simpler terms, everyone that's going to have a desktop already has it. There's growth in the install base as a person matures past childhood but this is offset by losses from those leaving the marketplace altogether.
As iPhonewarrior inferred, OSX (Mac's) aren't making a huge dent when compared to the number of Windows installation. The press is just hyping the accelerated growth in the OSX install base. Something that's easier to do when given the smaller number of existing installs.
Kind of like something I heard on the radio today. Regularly consuming vitamin E will increase the rate of prostate cancer in men by 17%. That seems like a significant number then you dig and find, the incidence rate is 122/100,000 and a 17% increase equates to ~21 additional incidents if all 100,000 men regularly consumed vitamin E. The reality then is most GPs will not see even 1 new case.
