They had caught extra fabric in the sewn seams and let them leave the work room like that. Without cutting the welt cording material on the bias, they'd also sewn the fiber of cord into seam, causing a great deal of puckering and wavery lines of uneven width. And what's with the black zipper against a light colored fabric?
The seams were not a consistent 1/2", because of running over the fiber cord and when the straight-grain corded welt didn't want to make a nice smooth circle, they sought to ease the seam by clipping. This doesn't work real well. A lot of people quote lesser amounts of yardage required and cut welt with the grain, and in a pinch, maybe - and okay for squares and straight edges, but not circular shapes or "T" or "L" cushions. You should just figure the extra necessary to bias cut your welt cording...you'll be glad you did. Still, some people don't like to do it.
Ah, that's more like it...
You should be able to crimp the welts circle like a pie crust into the sleeve of an already zipper-applied tube that is your bolster - without slashing the seam allowance for ease.
Begin pinning at 4 quarters of circle - east, west, north and south. If you've measured correctly it'll fit perfectly and you won't have to adjust a thing. The crimping kind of puffs the bolster's ends out and accomodates a larger-than-finished feather down cushion just right. You'll have a beautiful round bolster. And a similar colored zipper is always a good idea, unless something is absolutely not going to show.
Nice if the bolster tube will kind of stand on it's own at the machine plate - then there's no poking or prodding, only smooth guiding that results in a perfect bolster.
If you've sewn your welt cording on first, make sure that you sew just inside of your previous stitch - this way it won't show when you turn bolster right-side-out.