I don't understand. So, the chances of getting Jean in Venti's banner were higher than in Jean's banner?!
Yes, of course. The description of each banner contains information about the percentages.
Venti's banner = Venti himself + five other 5* characters + zero 5* weapons.
0.6% for 5* characters in total, of which 0.3% for Venti and 0.3/5 = 0.06% for each other character, including Jean.
Standard banner = five 5* characters + ten 5* weapons
we get 0.6/15 = 0.04% for Jean
The chance is exactly 1.5 times lower on the standard banner.

В тихой однообразной жизни всегда так бывает: каждый отдельный день бесконечно долог, но месяцы и годы мчатся с непостижимой быстротой.
I wanted to offer you some flowers, but a crown would suit a princess like you much better
Смысл жизни в том, что жизнь на тебе не заканчивается