ps -aux -U “user” not filtering on user as expected

I have a quick question. I would regularly like to list all the processes running under my account on Linux. After reading the manpage, I thought I’d have to write:

ps -aux -U "joshua"

to get all processes running under user account joshua. However, this command also lists processes running as root and other accounts. Am I missing something? Why does this happen?

Thank you in advance,
Joshua