I do think 'a disagreement on utility calculations' may indeed be a big part of it. Are you a total utilitarian? I'm not. A big part of that comes from the fact that I don't consider two copies of myself to be intrinsically more valuable than one - perhaps instrumentally valuable, if those copies can interact, sync their experiences and cooperate, but that's another matter. With experience-syncing, I am mostly indifferent to the number of copies of myself to exist (leaving aside potential instrumental benefits), but without it I evaluate decreasing utility as the number of copies increases, as I assign zero terminal value to multiplicity but positive terminal value to the uniqueness of my identity.

My brand of utilitarianism is informed substantially by these preferences. I adhere to neither average nor total utilitarianism, but I lean closer to average. Whilst I would be against the use of force to turn a population of 10 with X utility each into a population of 3 with (X + 1) utility each, I would in isolation consider the latter preferable to the former (there is no inconsistency here - my utility function simply admits information about the past).