My understanding (and it's relatively poor even though I take Concerta myself) is that the mg is the total dosage that will be gradually released from the XR capsule over the day - so for the majority of the day (especially in the morning) you're feeling far less than 18mg of methylphenidate in active use.
I think this is a discussion to have with your doctor but I'd suggest not coming in overly rationalizing - just tell them how you're feeling on the dose (i.e. it's less impactful) and see what they suggest.
Personally, I've found a different efficacy in Concerta vs generic methylphenidate XR at the same dosage and that tracks with my general impression of any neurological pharmaceutical agent... some things randomly work better for some people and violate common expectation or reason because brains are really fucking complicated, everyone is different, everyone has different brain chemistry, ADHD (and all other neurodivergences) are diagnosed by symptom rather than cause, and, most importantly, we really don't have a fucking clue why things are effective and the entire field of neurochemistry is really just vibes based.
I hope you find a good resolution!