Maybe chupas are likely to be missed in close-quarters with 122% accuracy because there is a voxel near their center-of-mass voxel which is empty. Think about it, they have that arching form, independent legs. Some other observations:
The stats here seem to suggest to me that cover and size will only be effective at blocking/dodging misses. Each deviation causes 0.08 "max deviation" so real voxel deviation, divided by 2 since it's split between negative and positive. In order for 0-10 "grazes" to cover the whole block, you would have to go to about 50 tiles, so that 5*50*0.04 = 10. Half the block might be more reasonable, at 25 tiles, but that's still pretty crazy range for just a 10% reduction. Theoretically 10% can be noticeable with machine guns that only have 10% CTH anyways.
However chance to roll a miss but hit anyway is significant at most ranges. With average "max deviation of 4", which again is really 2 due to the positive-negative split, at 8 tiles range you still have a 1/3rd chance to hit the target tile if you miss. And cover/size is totally effective at stopping that, since it can hit any voxel on the tile. So oddly enough cover is probably better for close range.
All of this is false if you have cover that's one voxel away from your center of mass though, because then it will block 33% of a hit and 50% of a miss. That's just very rare. Less rare when you kneel below cover that then rises above your gun. On that note, do aliens still try to shoot if they have no LoF to the center of mass? You know, a player can get the "No line of fire!" message but know it's wrong and choose to shoot anyways, can the aliens? I doubt it, but if so cover could be broken, and there was one mission in XCF where I felt like the cover was blocking the vast majority of the shots.
