Is it just because it is one of those words that is simply masculine?

Nouns in Spanish are simply masculine or feminine (although there are a handful that can be either (are hermaphroditic?). The problem is that someone told you that the "rule" is that nouns that end in "a" are feminine. This is not a rule; it's a generalization (and would be more precisely phrased as "most nouns that end in "a" are feminine [and most nouns that end in "o" are masculine]).

Grammatical gender is not a question of rules/logic but, rather, of custom/tradition. In many cases a noun's gender is whatever it was in Latin/Greek (and has nothing to do with the meaning of the word).

As a simple matter of probabilities, if you see a word ending in "o"/"a" that is unfamiliar, you would do well so assume that it is masculine/feminine (but that's only because it's the "odds on" assumption). If the generalization does not apply in a particular case, it is pointless to ask "Why?" (because the only real answer is, "Because that's the way it is.")

¡Hola!, Chobit:

Span¡sh D!ct also has a great Reference Article explaining the genders of nouns.

You will find it here ----> Gender of Nouns

If you go over to look at it, and I encourage you to do that, have a look at what is in our Reference Library. There is a lot there that you are going to have to deal with in school. It really is a treasure trove and the Index Page has links to a huge amount of information. I suspect once you know about it you will return often.

Here is the Index Page ----> Span¡shD!ct Reference Library Index

Recuerdos/Regards,

Moe.