I've just given the image in 306 a critical eye examination myself and it is really off (although I never really noticed it before it was brought up). Long post ahead.
(The lines seem to be diverging as the sheath is not a perfect rectangle but a curved one, the lines would re-converge).
It doesn't seem to be curved as we can see quite a lot of the sheath at the bottom and it doesn’t even begin to curve, like it would. Curved swords, like katanas, are curved in a kind of "c" shape with both ends of the sword pointing away from the blade. If this sheath is curved it would be curved in an "s" shape, with a change of direction in the middle as both ends are pointing in the same direction i.e. one away from the blade and one pointing towards the blade.
Being an amateur artist myself I can tell you right now that drawing two interconnecting parts of something that appear separate isn't hard at all, its quite easy if you do it right. Any professional artist doesn't draw in ink straight off the bat, they draw in pencil and then go over it in ink. So all Kishimoto would have to do would be to draw the full object but then not ink the parts that are behind Sasuke. This is the preferred method just to avoid the kind of problem we seem to have spotted here.