Why didn't I get the Army of Four achievement in XCOM: Enemy Unknown? Did I miss something?

Today I finished my 3-day long, Classic+Ironman run and aimed to get these 4 achievements: "Earth First", "Ain't No Cavalry Comin'", "No Looking Back" and "An Army Of Four". All my soldiers lived until the very end (I did change one Assault for another and made him the one, my Heavy was the one I kept in all missions) and I got all the achievements except for "An Army Of Four". My only fails were taking down the UFO Overseer twice. I constructed the Officer Training School late in the game and bought all the trainings except for "Squad Size I" and "Squad Size II". I did cheat Ironman with closing the game and restarting to improve every turn, but sometimes it took me back to the situation room right before accepting panic, abduction, etc. missions. (Maybe that has something to do?)

Now, the achievement in Steam says "Beat the game without buying a Squad Size upgrade (Classic+ difficulty)" I've done some Google research and all I've found are the descriptions of the said achievement.

My question is, why I didn't get the achievement? Probably I missread it, or did something not so obvious to not get it.

Solution 1:

An Army Of Four is one of 30 achievements added in XCOM Enemy Within. These achievements will still display on the Steam interface but can't be completed without owning Enemy Within, even if you do what it tells you.

Solution 2:

If you do not have the expansion Enemy Within, the 'Army of Four' achievement is unavailable and thus you can't get it.