There would seem to me to be NO mystery as to why the deformation occurs more in the rer pellets than the front ones.
When he shell is fired the pressure "HITS" the shot in the rear. The rear shot have the weight of all those in front of them to move while the front ones are only pushing air.
Take a lead bar & lay it down on its side. Smack one end gud'n hard with a hammer. see which end is swelled, the end where you hit it or the other.
Forget all the scientific theory that has been spouted & use your brain a bit. Tom's experiment that Stan has quoted has verified this but this is what would have been expected all along.
That 1400 something psi per Brinell hardness number I quoted earlier for the deformation of lead is actually 1422 psi. I first encountered this in reading about cast bullets in revolvers where it was stated the bullet should not be so hard as to resist bumping up to seal the chamber throat. Not sure about the spelling but the 1422 psi was cited from Veral/Vernal Smith. A bit of research shows this is actually the figure in determining Brinell Hardness which is based on Kilograms per Sq MM. 1 Kilo per 1mm converts to 1422 lbs per Sq In. thus if you smack the shot with a 10K psi load the shot will have to have a Brinell Hardness of 7 to resist deformation.
