Honestly StarCraft's Marines seem more inspired by Starship troopers than WH40k. They are called Terran's after all. The Marines also aren't knights.
Earth is Terra in 40k also.
From earth-> Human = Terran
Space Marines are not Knights.
But all and all it will all come from what you were exposed to when you were younger.
Those that grew up in front of a Nintendo, think Samus
Those that grew up in front of a PC will think Starcraft or Doom or who knows what.
If you grew up reading comics and actually were one of the few that read Iron Man (it really was not popular) then Iron Man it is or perhaps even one of the other battle suit comic characters.
Cartoons have their own.
Tabletop and pen and paper have theirs.