Active Ingredients: Caffeine (200 mg), Alertness Aid.
Inactive Ingredients: Carnauba Wax, Colloidal Silicon Dioxide, Croscarmellose Sodium, D&C Yellow 10 Aluminum Lake, Dicalcium Phosphate Dihydrate, FD&C Yellow 6 Aluminum Lake, Hypromellose, Magnesium Stearate, Microcrystalline Cellulose, Polyethylene Glycol, Polysorbate 80, Starch , Titanium Dioxide
So unless one of those "inactive ingredients" is actually not so inactive (I don't know what they all are), it sounds like probably quality control and/or timing and/or placebo.
Off-brand caffeine tablets are unregulated, but Vivarin is FDA-approved. My guess is it's mostly down to quality / quality control.
Tolerance wouldn't factor in if you're taking supposedly-equal doses of other brands yet experiencing a weaker effect.