Sneeuwgors

The very first pic I took on Svalbard was of this male Snow Bunting. One I  would have thrown away, if it weren't for the upperparts of the bird. With the rump and lower/central back all white, it matches the description of ssp. vlasowae rather than nivalis (that should breed here).  It does seem to lack largely white uppertail-coverts, so probably this is just variation of nivalis I was unaware of? After this bird I checked many more, but none showed such extensive white upperparts. I hardly ever get to study Snow Buntings in summer plumage, so comments appreciated.