Addressing someone by their initials?

This could go in the Work, Bitch group too, but decided to post it here. I have been corresponding with someone for a few weeks, so I try to do the polite thing and give them at least a week in between follow up emails. Today the person replied by saying "Hi EB"... not "Hi Everything" and I thought this was so strange! I feel like if I did this to someone I would definitely be annoyed at them...