I don’t think your conclusion is correct. First of all, your experience mirrors mine, I cannot reach max heart rate during ramp tests. But I have had ramp tests where I have given everything I have had (e. g. I spent 2–3 minutes on the floor, panting), and yet my heart rate was 12–15 bpm lower than my max heart rate.
My understanding is that during a ramp test your limiter is oxygen uptake and not maximum cardiovascular performance.