Games Workshop TT Space Marines could never EVER be any where as tough as they are supposed to be, simply for the fact they have to balance them out with every other race type in the 40k universe for a table top game, and thus table top has no bearing on RPG Space Marines, because in a RPG Space Marines can be exactly as tough as they are supposed to be, simply because they are not balanced around making a game viable.
A RPG has a much broader spectrum for characters, and their capabilities, a table top game is no where near the same medium.
As for why SM weaponry does more damage, they have a significantly higher grade of weaponry and Ammunition, Stronger metals used in bolt tips, higher grade propellants, more powerful chemicals for flamers etc etc, this is why they do more damage, this is why there Armour is so superior.
Imp guard and normal humans do not get to use such gear because it costs to much, it is a bad investment giving un-augmented humans such weapons and armour, because they die to easily and it is that simple.
Not once has the economics of the system been brought into this discussion, yet it is the one thing that clearly gives a delineating reason for the differences some of you are struggling to reconcile.
Space Marines have been designed to be self contained armys, with the very very best gear that can be manufactured for them because they are a good investment, they are in comparison to a standard human, basically another species, they are godlike without the gear they have access to.
Imperial guards are dog soldiers and they get gear that reflects it, mass produced with the cheapest materials that will be sufficient to make them work, the exact polar opposite of what Space Marines are and get.
I have zero issues accepting a SM bolter round for round is dealing more damage than an Imperial guard HV Bolter, because although they share the same names, that is the ONLY thing they share, and that is what seems to blinding some of you, just because it has the same name does not mean it is the same, Car means vehicle does that mean all Cars perform the same? no it does not, different materials are used, different price ranges aimed at different users, 40k is no different.