It could simply be that the old RCD was near maximum sensitivity and the new one near minimum. A 30mA RCD *must* trip at or below 30mA leakage but may trip all the way down to 15mA and still be within specification.
A large installation (as ours was before I replaced consumer units and installed multiple RCDs) may well cause nuisance tripping of a single
30mA RCD simply because of its size. Even when all insulation resistance etc. is within specification if there's enough little bits of leakage they will add up to something that trips the RCD, especially on wet days and/or when a number of PCs are turned on.