Do you want to know a secret? People make mistakes. They don't mean to, and often small details are left undone. Take this stone for instance:
This one came to me already transcribed in my queue today. Supposedly, this was already done. However, this stone when I got it anyway, had his whole name placed in the first box. This stone also clearly has three names written on it. The original transcriber only placed one name, and forgot to add the other two. So, what did this do for me? I was able to get credit for all three transcriptions even though this was supposedly done already.
So why do we have people go over stones. already "transcribed"? For quality control, and to better help people with their searches. Plus, you may get to catch a mistake that really does make a difference!