Question:
Why are Windows NT user profiles created with extensions of .000, .001, and so on?

Answer:
The following is from the MSDN article Q185588, “Guide To Windows NT 4.0 Profiles and Policies (Part 3 of 6)”:

“Should multiple users with the same account namelog on to the network, separate distinct profiles are created for each. For example, ifmultiple users with the account name John Smith log on to the computer, the first JohnSmith is assigned a folder named JohnSmith. Subsequent users with the same name areassigned folders named JohnSmith with a numerical suffix appended, for exampleJohnSmith.000, JohnSmith.001, and so forth.”