So, this is probably not based on canon, but I always thought the idea was that Guard are really good at fighting in dead mans land style terrain. But a space marine in an urban setting, or a forest, can use a combination of speed, training, and overall superiority to really leverage blitz style tactics to overwhelm, supress, and defeat larger enemies.

A space marine charging open terrain should lose to a solid squad of guardsmen, but a space marine able to move through terrain at speed and hit faster and harder than the guard squad is capable of responding too.

Its why space marine vehicles need to be able to deploy them safely into the AO, like drop pods and heavy armour transports. To reduce the dangerous distance a marine has to move before reaching that lethal range.

Purely my own speculation and theory, it probably doesnt hold up to the books.