Aunque parezca mentira

Aunque parezca mentira
 

Hi,

It means either believe it or not or oddly enough. Why subjunctive?

This is a set phrase (frase hecha). It always appears in the subjunctive.
Other ways to say the same thing may help you to see that it is introducing an idea that is incredible (not believable). Something that can't be real, or things that seem impossible, will be said using the subjunctive mood.

aunque parezca mentira
aunque sea mentira
aunque parezca increíble
aunque sea increíble
aunque parezca raro/extraño
aunque sea raro/extraño
(por) increíble/raro/extraño que parezca/sea
parece mentira/increíble/raro/extraño que ...
(the clause that follows will contain a verb cast in the subjunctive mood)
increíblemente, etc.