The warning flag for this puzzle is evident in the intensities of the signals.
For an unknown compound, the TOF-ESI+ mass spectrum below shows the expanded region around four isotope clusters at m/z 300.336 ([M+H]+), 322.307 ([M+Na]+), 599.672 ([2M+H]+) and 621.611 ([2M+Na]+). The intensities of the 13C isotopes (i.e. the [M+1] signal) vary between 40 to 100%. This is an indication that too much sample was injected. Detector saturation can lead to inaccurate mass measurements. The next step is to dilute the sample, recollect the MS data and check that the signals are of sufficient intensities.