From LHCb results summary https://lhcb-public.web.cern.ch/Welcome.html#Deltamc :
(m1-m2)/(D0mass) = 3x10^-15
10^-38 is an absolute measurement, not the ratio.
(m1-m2)/(D0mass) = 3x10^-15
10^-38 is an absolute measurement, not the ratio.
Besides, relative measurements are often easier than absolute ones, so even if you could determine that the masses of two objects of ~1g differed by 10^-38g, that wouldn't necessarily tell you the absolute masses to anywhere near that accuracy.