Q: "Your Apple ID can have up to 10 devices and computers (combined) associated with it." Then why does iTunes not let me authorize any more than 5 devices?

Probably because of the GUIDs. When you upgrade or change hardware this tends to happen, especially on PCs.

Check with

iTunes Store Support

http://www.apple.com/emea/support/itunes/contact.html

I have the exact same question...when I try to turn on iTunes Match on one of my computers, it gives me the error: "You have reached the limit of 5 authorized computers." My "Manage Devices" list includes iPhone and iPads (as well as Macs). If the limit is actually 10 computers and devices (combined) per HT4627, why do I get this limitation?