Gender identity is definitely different than expression. I think people feel the best when they are able to express their true gender identity. I'm not sure I can think of a case where expressing the opposite is preferable. When not cis, Because of gender roles and stereotypes, being able to express the preferred gender isn't always an option, and unless you are able to express it outwardly enough, people don't even notice it. For example, I can have no facial hair and long hair, but nobody has a clue I'm trans and trying to express androgynous or feminine look because of other subtle ques. It is even harder in some ways for FTM before HRT, but usually easier for FTM to express a male gender identity after HRT.
As Cindy mentioned, identity is a spectrum and you may fall on one side or somewhere in the middle. Maybe it is on one side closer, but you may choose to present even more towards that binary than you really identify for various reasons. How you present is basically gender expression, which does not always line up with identity.