I have a feeling that JC might have added the extra 2 traces empirically, in an attempt to reduce the impact of the "blockmap bug". In other words, at certain distances from the shooter, the side traces just might enter the adjacent block. This gives a shot that would normally "slip thru the cracks", a chance to work anyway. If you think about it, it's kinda good that that blockmap bug existed, cause we ended up with a game that spreads pellets semi-realistically, providing that "real-world" feel that would be difficult to otherwise fabricate, IMHO.
Of course, it's quite unrealistic in fact, and it makes it quite difficult to shoot the SSG, and have only a few pellets hit your target, since your shot is bent towards the center. I think the game would feel better without the horizontal auto-aim - I think I'll mock it up and see for myself: 0 extra traces, 2 extra (original), 8 extra, etc.
At any rate, I find it both humorous and tiresome to see the instant "Don't tread on my Doom" holy war that erupts from the suggestion of creating a simple, toggle-able setting. Does anyone really believe that a programmer worth his salt would, intentionally, irreversibly, hack up such a fundamental aspect of their favorite game? Or the bunch: "Ugh, Doom is so broken! We NEED this change, or I'll never be able to fire the SSG slighly off angle again!". Doom. We know drama.