It’s possible some of the variation is from the specific manufacturer or manufacturing process. In the case of Wellbutrin XL, generic brands have even been recalled due to not being bioequivalent (https://www.medicinenet.com/script/main/art.asp?articlekey=1...)

I have been told by a doctor that effects appear inconsistent across brands or places of manufacture, even with non-recalled generics. You should always consult with your own medical professionals about your medication, but you can often ask for a different generic at the pharmacy.