"PO_Neptunus" doesn't have as much exposed white hull as "US_SotL" two spaces to the left, and anyway it's not the white which is bright, it's the yellow upper hull. But it does have a different sky to most of the others, so my guess is that the screenshot was taken at a later time of day so the sun is low and shining straight at the ship's side. If I'm after a screenshot, I usually save game while on worldmap, wait until it's about midday, go to 3D sailing and hope it's clear weather. If not, I switch between worldmap and 3D sailing a few times in the hope of clearer sky before it gets too late in the day, and if a storm shows up then I reload the savegame.
The point is, "PO_Neptunus" looks different to surrounding portraits and has been accepted for years, so it's evidently not a major issue if a portrait doesn't quite match its neighbours. There's also a fair amount of variety in ship orientation, proportion of sea to sky, and colour of sea, so when getting the screenshot it's not essential to get precise positioning and lighting. (There's another portrait in one of the other files which was definitely taken near sunset because the sun is visible behind the ship. And the shot of Sao Feng's junk probably needs to be redone, partly because it shows basic pirate flags rather than the dainty personal flag for Sao Feng, and partly because it's facing the wrong way. )
The point is, "PO_Neptunus" looks different to surrounding portraits and has been accepted for years, so it's evidently not a major issue if a portrait doesn't quite match its neighbours. There's also a fair amount of variety in ship orientation, proportion of sea to sky, and colour of sea, so when getting the screenshot it's not essential to get precise positioning and lighting. (There's another portrait in one of the other files which was definitely taken near sunset because the sun is visible behind the ship. And the shot of Sao Feng's junk probably needs to be redone, partly because it shows basic pirate flags rather than the dainty personal flag for Sao Feng, and partly because it's facing the wrong way. )