Maybe the better question is, why is Kellerman the only person who talked about a "flurry of shells"?
Yes. And in his same day official report, he wrote this: "We were still traveling at the normal rate of speed of from 12 to 15 miles per hour when I heard a noise, similar to a firecracker, exploding in the area to the rear of the car, about 12:30 p.m.
Immediately I heard what I firmly believe was the President's voice, "My God, I'm hit!" I turned around to find out what happened when two additional shots rang out, and the President slumped into Mrs. Kennedy's lap and Governor Connally fell_to Mrs. Connally's lap. I heard Mrs. Kennedy shout, "What are they doing to you?"
Three shots. No "flurry of shells".
Which supports his testimony where he said the "flurry of shells" were two shots.
Mr. SPECTER. Now, in your prior testimony you described a flurry of shells into the car. How many shots did you hear after the first noise which you described as sounding like a firecracker?
Mr. KELLERMAN. Mr. Specter, these shells came in all together.
Mr. SPECTER. Are you able to say how many you heard?
Mr. KELLERMAN. I am going to say two, and it was like a double bang--bang, bang.
Again: "I am going to say two"..... His "flurry of shells" equals two.
Kellerman report:
http://mcadams.posc.mu.edu/russ/testimony/sa-kelle.htm Kellerman testimony:
http://mcadams.posc.mu.edu/russ/testimony/kellerma.htm