Different manufacturers can use different non medicinal ingredients in their formulary. I know a woman who is really sensitive to meds. She takes an antidepressant that's available as a generic. The pill from one pharmacy works great for her, but when they discontinued manufacturing she found that none of the pills from the other manufacturers worked for her.

Also sometimes people react differently to lower vs higher doses of meds. For example I find 25 mg of Seroquel much more sedating than 200 mg, which makes no sense to me, but that's what it is for me.