Correlation between Email-id and Tenure

I noticed that there is a patten in a lot of emails that we get everyday. 

In most of the cases, you can tell when did the sender joined the company just by looking at his email id.  

And the pattern looks something like this:
 

Stage of company          Email id of sender

  Drawing room office      nickname@company.com                

  One room office            firstname@company.com
  One floor office             firstname.lastname@company.com
  Too late                        lastname.firstname@company.com

Have you noticed something similar?