Because if the same hardware ID is being used by multiple users, and Steam allows multiple users on one PC without any issues, that's in fact over 100% in total. To colour the example, if everyone surveyed have three accounts in their Steam and they logged in to all three to send results, in total that would be 300.00%.

Yea... Someone on Videcardz also pointed it out with the following example:

Processors:
Intel: 65.62 %
AMD: 36.76 %

Total sum: 102.38 %

The total of the reported percentages for many of the categories add up to a lot more than a 100% this month though. For example, Video Card Description: pc adds up to 129.77% this month, when it's usually more or less consistently 100%. The relative distribution is probably correct, but it does make me wonder how much we can actually trust the reported numbers.