Hate to be the bearer of prejudice, but your 'seeing it' is notoriously... how we say... 'out there' Lux.
The game is easy enough to discount as canon: gameplay sacrifices story.
A heavy stubber is the equivalent of a 50 cal hmg. That means a Guardsman can punch as hard as a .50. One round to the chest just disintegrates the standard human, yet it takes 2 hits to reliably, mathematically kill a guardsman by wounds, and 33% of those shots are saved, essentially takes 6 hits to reliably guarantee a kill of a guardsman with a 50 cal gun. A guardsman is, admittedly, an immensely fit, highly skilled individual (according to Cadia and Tanith as example, likely around Marine or even SEAL level fitness and training and/or experience), but NOTHING human can withstand a big 50 direct hit.
And yet they do.
Not to mention in close combat they punch with the same force.
Game = canon? I think not.
You're comparing 40k table top to Halo a digital game, the reason the Halo digital is 1st degree canon is because the books came after it and were based upon it.
It is similar to the varying levels of canon that George Lucas classifies for the entirety of the star wars mythos.