Why not? What specifically precludes those fragments from having come from CE-399?
The basic facts are.... the nominal weight of the bullet is 160 grains.... but due to manufacturing tolerance it could have weighed 159.5
CE399 obviously lost some weight in the barrel of the rifle..... which would put it's weight at approximately 159.6 at the muzzle.....CE 399 weighed 158.6
Which leaves only one (1) grain for the fragments..... and the fragment in Connally's leg was probably heavier than 1 grain....