I have run picrust2 pipeline for my smaples and I notice that the sum of 'norm_taxon_func_contrib' for a given function is not equal to 1. am I missing something here? Can you please clarify how this is being calculated? Our understanding is

'norm_taxon_func_contrib' = taxon_fn_abun / (sum of taxon_fn abun for the given fn)

Thank you.