It’s something of a legacy of the game’s design process - in order to keep things simple, some aspects of weapon stats from earlier drafts were simplified as we iterated the rules… and in the process, some of the mechanical differences between similar guns were lost. In this case, an earlier draft had rate of fire mechanics more similar to the way it works in Mutant Chronicles/Infinity and in Fallout (where guns have a specific RoF value which caps how much ammo they can spend and the bonus gained from doing so), where semi-auto rifles like the Garand had a lower damage but higher RoF than bolt-action rifles like the SMLE (but there was a talent that let characters boost the RoF of their gun because they were skilled at reloading, to reflect things like “Mad Minute”)
There’d be nothing wrong with just standardising all three to 5 damage dice as a house rule.