To allow easier retries, and to prevent breaking in odd cases. You have multiple quick save slots, so this should not cause any adverse issues for readers.
He didn't at first during that phone call and still doesn't even really know who she is? He explicitly has no idea Zach even got a girlfriend, and Zach had to explain who she was, but then after Zach explained how he had fixed up her car his dad had an idea of who Britney was even though his wife had only briefly mentioned Britney to him a single time.
Maybe I am misreading this question, but over the first third of the only phone call Zach has with his dad so far is Zach talking about Britney to him. Maybe you meant the call with his mom?
Samantha (Zach's mom) is almost supernatural cognizant due to her staggeringly high intellect, deductive reasoning skills, and ability to read people. She pieces together Zach is talking about Britney when he explains that he got a girlfriend he knew in high school before he even finished his sentence. The clues were fairly obvious to her (Zach was clearly simping for her in high school, Zach kept bringing her up to his mom, Zach reduced how much he was studying to help her with theater club as a tech member, they hung out most days at lunch, many times after school for theater club, and a few times during the summer), and Samantha has a compulsive need to flex how smart she is on people.