Why are there different forms when registering gmail?

Question

Google Chrome

Hello!
Yesterday, when registering an account, after entering the name, surname, mailbox name and password, I was then transferred to the next page, where there were several fields: phone number (OPTIONAL), backup email (I don’t remember exactly, but it seems also optional) and there are some more fields.
Well, I figured out what it means that you can register an account without a phone number (like from an android), and since I haven’t decided on the name of the mailbox yet, I decided to postpone it.
Today I composed the name of the box and decided to regnet, but at the second stage, so to speak, I was thrown to a page where you only need to enter a phone number, there are no more fields.
What is it connected with?
Thank you.
ps has already come across such a form and registered several accounts without a phone number.

Answer 1

I think we are dealing with a random. That is, some part comes with a check, the other does not

Answer 2

What is it connected with?

here with this

ps has already come across such a form and registered several accounts without a phone number.