The term crunchy here mostly applies to the combat rules. When making attacks you follow a lot of steps, sometimes multiple times (tracking the damage and location of every single bullet for example). So I guess in this case it means "procedures heavy" more than "math heavy" because, even if calculations are done multiple times for a single attack, the math is very simple (mostly adding or subtracting low numbers).
Overall in the game I tried to keep complexity as low as possible (therefore I used the term "streamlined" to describe it), but I kept these rules in because I liked the "realism" and danger that they brought to combat.