It seems quite simple to me: Either Hillary is too stupid to recognize classified information when she sees it, or she was illegally hiding all her emails from FOIA requests. There are no other options.
Bingo
Apparently she had underlings scan and remove the 'Classified' header so she could deny sending that type of material. Underlings handling Classified docs !.
Knowing the Clinton's and word parsing she may not be counting Top Secret and Your Eyes Only docs when she says she never sent Classified e-mails. I'll bet that is part of it.
OTOH she must be pretty stupid to not recognize sensitive material.
Either way, she is unfit.