Consider this. Short fat case with a pointed bullet. (or even a longer case) You're single feeding so you drop the round in the solid bottom action opening. You start to close the bolt and the bullet hits the area around the chamber opening, rather than starting into the chamber.
You know what a coned bolt looks like? The bolt head is tapered back and the tenon of the barrel is tapered to match the bolt face. Like a pocket or funnel the bolt fits into. So, the tapered part of the barrel tenon acts like a ramp that the bullet hits and starts it "up and into" the chamber. Look up "coned bolt" and you'll get the picture.