不一定是...

看起来精明 不一定是 真的精明

(人很精明) ── Previous context

“Looking shrewd doesn't necessarily mean being shrewd.”

但凶手 不一定是 戴面具的人

(有卖是有卖) ── Previous context

“They do, but… the killer might not be necessarily wearing a mask.”

三婶 不一定是 蜂王浆的事

(喝这玩意儿啊 闹肚子) ── Previous context

“Third Aunt, it might not be the royal jelly.”