Probably they had some research and found out that people less like to register because it sounds boring and more like to sign up because it ... something. )
Maybe you could have a notice explains that register means the same thing as sign up. That way not only do you get a new member, but you get one who is better educated because they just learned not only a new word but one with more than two syllables.
It may also help if they want a bank account one day because they will know which button to click on or tap.
Probably they had some research and found out that people less like to register because it sounds boring and more like to sign up because it ... something. )