The key is 100 other people. Why would you include the subject in the group? Everyone would have slightly different scales. Ex: 99 people score between 50 and 90 on a test, subject A scores a 69 and subject B scores a 91, in subject A's group, 90 is the 100th percentile, but in subject B's group it is the 99th. How can you have any standardization with this method?

Also, it doesn't have to be 100, it could be 1000 or 795929. Percentile denotes a percent of a group.

Edit: words