It only matters if you want to get the details of the science right. The early studies confirming the link between DDT and bird deaths tested for the presence of DDE in eggs. DDE also appears to be the causal agent responsible for eggshell thinning by inhibiting prostaglandin synthesis in birds' uteruses, thereby disrupting calcium transport in the eggshell gland.
I posted links to two excellent recent articles about Rachel Carsons before, but here they are again:
I sort of understand this, but it seems like a distinction not worth making. If DDE is generated by DDT, then what's the difference?
DDE is a metabolite of DDT—i.e. is produced when DDT is digested.