Two things at work here:
1. While Apple does a good job with CS, their customer base is small compared to the total of all the other PC companies their measured against, AND Apple customers are known for being fiercely loyal. Translates into a high concentration of positive reviews.
2. Apple is the only Mac vendor. They have the luxury of having most, if not all, parts made to their own specs. Some parts may be made better (since a Mac is so ******* expensive), and they know everything will work well together because of their closed "system". A lot of computer problems are caused by operating system/software/driver problems. Apple controls the integration, translating into fewer problems.
On the other hand, PCs have multiple vendors, with some doing a good job, some not, which tends to taint the whole PC platform perception. More importantly, if a particular vendor, say Dell, decides they want to really do something about reliability, they can only control customer service/tech support people. While they can tell actual manufactures of the internal parts the specs they want, because they're only assemblers of mass-produced parts, they can't totally control reliability or how they work together. And while they can suggest things to MS, the OS experience is pretty much out of their control too.