Horned grebe's forehead (1 Viewer)

[JayFeatherPL]

People say that a great way to separate a horned grebe from a black-necked grebe in winter is to look at their foreheads. Horned grebes should have a slightly sloping forehead and the crown should be generally flat, while black-necked grebes should have a steep, almost vertical forehead. And that's true to most grebes, but take a look at this horned grebe (the first picture). It has a rounded head, not flat, with a steep forehead just like a black-necked grebe. Why? Why some horned grebes have a bookish, slightly sloping forehead and a flat crown and some have a steep forehead identically to a black-necked grebe? Actually, while looking at the black-necked grebes' pictures in the internet, I haven't found any black-necked grebe with a flat crown, so it's only a horned grebe which can have a similar head to a black-necked. I also upload a black-necked grebe to compare. Is it actually possible to ID these two grebes by their heads' shape?
The pictures aren't mine! The author of the first picture is Becky Matsubara. It's taken from this website: Horned Grebe
The second picture is taken from the BTO website: https://www.bto.org/understanding-birds/birdfacts/black-necked-grebe

 

[Butty]

Head shape can be very useful but varies. More-constant features of black-necked grebe in non-breeding plumage are fuzzy (rather than sharp in Slavonian grebe) black/white division on cheek and uptilted-looking lower mandible.