Doesnīt matter if itīs a sunnen or a mitutoyo diallbore gauge from precision or performance point of view...sunnen use the measuring finger and a lock nut to adjust the zero on the dialgauge...others use fixed fingers and adjust the height of the dialgauge.
Not sure why a boregauge with fixed fingers(washers) would be any less accurate...tell that to mitutoyo
For measuring inside a round hole and not just checking it against a std a 3-point micrometer is the way to go !