It's worth nothing that she starts at 60 AD early due to the 20% TAD on-hit she has. Her base AD is set to 50 to account for the extra 10 on-hit damage she's getting.
This thread was added on July 21, 2020, with posts from RiotAugust.
Original Post
/r/sennamains/comments/hv4ppv/tedx_how_many_souls_does_senna_need_to_keep_up/
Does the %tAD on-hit exist to increase/gate her scaling with sources of AD/her scaling with bonus crit damage?
Yes to some degree. It exists to lower her crit synergy and increase her synergy with flat AD purchases. Her attacks essentially have a 1.2 TAD ratio when you're buying dirks and longswords.
Also, it being an on-hit means it applies on her Q to champs but not to minions (meaning her Q is worse at waveclear)