Meitnerium has no isotope that occurs naturally: every one of the 15 the tables list is made in a reactor or an accelerator, so the element has no standard atomic weight.
A mass spectrum does not show that average: it shows one peak per isotope, at the exact mass below, in the ratio of the abundances. That pattern is what identifies the element in a spectrum.
The periodic table, read every way a chemistry course asks for. This tool needs JavaScript; these are the pages it offers: