This is another issue related to composition normalization, which has to do with phase diagram construction. For some more background on this, please see my recent post about the calculation of formation energies:
According to the documentation of the PhaseDiagram.get_decomp_and_e_above_hull method, it returns the decomposition coefficients of normalized compositions. Normalized compositions are represented as having one total atom. For example, B2S3 becomes B0.4S0.6.
This means if you want to write the decomposition of B2S3 into BS2 and BS, you would actually write:
B_{0.4}S_{0.6} → 0.60\ B_{0.333}S_{0.666} + 0.40\ B_{0.5}S_{0.5}