I'm reading a book about discrete math written by Kenneth H.Rose and in it he states that in mathematical logic,
A proposition is a declarative sentence (that is, a sentence that declares a fact) that is either true or false, but not both .
Isn't it redundant to use the word "declarative" here? My reasoning is that any statement in English that is either true or false must syntactically be declarative. I'm feeling anxious though, because I think I'm missing out on some meaning and don't want to misinterpret this statement.
 K. H. Rosen, "The foundations: Logic and Proofs" in Discrete Mathematics and It's Applications, 7th Ed. : Mcgraw Hill