Wing patterns of Black-headed Gulls

This Black-headed Gull, Chroicocephalus ridibundus, clearly shows the white triangle on the far wing. This is supposed to only cover four feathers. However - it does look as though the outer web of P6 (as well as P7) is actually white rather than grey. Most books talk about the white triangle only extending over P10-7, the four feathers stated above. Note however the wide spread of the wing - normally the grey portion of these webs might be hidden? 

This bird also looks as if P6 is at least half white. It doesn't look too rare to me?

Here again the outer half of P6 looks white to me, at least on the right hand wing. And I still haven't understood how the underside of the wing can differ from the upperside:

This bird on the other hand appears to be entirely white on P10-P7, and nothing after that!