Subverted: Alice is just posing as Bob's perfect love interest because she doesn't think he'll accept her for who she is.
Double Subverted: Alice realizes she actually does like all the same things as Bob, thus they are a perfect match.
Alice is a female clone of Bob created by a mad scientist to be his perfect love match.
All of Bob's previous girlfriends all merge together to make themselves into the perfect girlfriend.
Zig Zagged: Alice is posing as Bob's love interest (see Subversion above) and tries to become the perfect woman for him but he really likes girls who are the opposite of what he is into so she tries that as well but she and Bob still aren't compatible and she tries the Subversion example again and it does work this time.
Averted: Bob and Alice are just friends, despite being very compatible.
"We need a girlfriend for Bob who will be his perfect match".
Or alternatively, she was written that way. So that fangirls could use her as an Audience Surrogate.
Lampshaded: "Isn't it strange that Alice popped into the town where her perfect match lives?"
Bob puts out an advert for girls that would be his perfect match.
Alice stalks Bob to figure out what his ideal woman would be like, and transforms herself into that woman, discarding her original personality, personal interests, and even changing her appearance.
Alice doesn't love Bob, but Bob has something she wants. She uses Bob's uncontrollable infatuation for her to advance her personal goals.
Someone forces Alice to act like Bob's perfect woman as part of their own scheme.
Defied: Bob refuses to go after girls who seem like they're perfect.
Discussed: "Alice seems to be a perfect match for Bob. They should date."
Conversed: "Alice on Teen Drama Show #57 seems like she was brought in just to be Bob's perfect woman"
From a distance Alice seems like she's the perfect match for Bob but once he dates her they are incompatible and he realizes that she is not perfect.
Alice's only trait is liking Bob, and her other hobbies that match Bob's are solely for getting Bob. She otherwise has no depth at all.
Alice reinvents herself for every guy she dates, adopting his likes, dislikes, beliefs, values, etc. to the point where she no longer knows who she is or what she likes and doesn't know what to do with herself on the rare occasions when she doesn't have a boyfriend.
Alice actively invokes this because she's immensely lonely, either having few close friends or a seriously messed-up family (or both). With Bob being her only healthy relationship (or one of very few), she thinks she has to be perfect because she can't stand the thought of him leaving her.
Reconstructed: Bob decides he likes Alice despite the fact that she's not perfect and they still make a good match.
Played For Laughs: Bob is convinced that Alice is too good to be true and various comedic situations are set up where he tries to prove she's not perfect. He is disappointed.