Presumably this will only be his share - there'll be a split with the rest of the band, potential sample clearances and any songwriting/publishing splits as a result of that, management fees, label recoups. And whatever deal Portishead signed for digital rights with their label will have a big old impact as well. If they've taken a big advance and given away their digital rights then it might just be that they signed a bad deal. Figures I've seen show a split between labels and streaming services as 70/30., this is a long way short of that but I'd guess there are reasons for it.