It's been awhile since I've played starcraft (that game is just way too fast paced for me to enjoy playing). But something else to keep in mind is that direct unit to unit comparisons in a vacuum aren't really all that valid. There are other supporting units that can come into play, and even other factors entirely.
For instance units that while weak in direct combat, might have other opportunities that stats can't really show (hit and run, harassment, scouting, the ability to pin back an opponent, force them to do something, divert their attention/micro, or require extra micro on your part to work properly). So balance isn't something that can be wholly looked at from a quantitative point of view.