Zephiel doesn't have a free QR2 (Armads is just a really, really good weapon and propels Hector in S, vs. a mediocre Ecksacks with a C-slot skill which just isn't anything special all in all on such a slow unit) for a much coveted B-slot on top of being able to run Wary Fighter. Hector is the only armor that reaches S, and few can even reach A.
Ninian is not S because Ninian sucks at anything but support, her enemy phase is just piss poor. She has a weakness to Red because of Falchion, rendering her ability to tank key reds mute, and she's also too slow and frail to tank many of the good reds anyway without an investment into TA3 AND Swordbreaker making her simply worse than Azura.
SCamilla needs an entire flier emblem team and is slow as sin.
Look at Eirika's stats, her only deficit is her attack (partially mitigated by 16mt weapon) and an amazing buff bot with an excellent speed tier. SCamilla is in a garbage speed tier.
Getting into S tier for our list was *very hard* as can be told by the lack of more than handful of units there. There needed to be a solid argument put forth, articulated, and defended for anything in S.