LM, the black eyes are the difference. It means she's got the 'spotted' genes and it seems as though the tup had one, which was hidden by his 'solid coat colour' gene.
What I know of Shetland genetics is that there are three 'places' for genes. Place 1 can have in it these genes, roughly in order of dominance - ie if you've got one higher up the list it'll cover over one lower down: white, grey, katmoget/gulmoget, other (non-agouti officially). The next place has genes for either black or brown - black is dominant. And the next place has genes for spotted or whole coat, whole coat is dominant.
So your white ewe's genes, despite her looking white, are probably: 2 x other, black and (black or brown) 2 x spotted. It's just that the black has been restricted to round her eyes, by the spotted gene.
And the tup will have been: 2 x other, 2 x brown, whole coat + spotted
So you've got lambs who are 2 x other, black + brown, 2 x spotted.
Hence their seriously cute socks and crowns
Mind you, I've got a ewe whose lambs don't fit the above - it's never as simple as it seems with genetics!