Those were not sable, almost certainly. Like moustress alluded to, they were probably brindles with very poor stripes that blended in with the rest of the yellow coat and caused the "different colored head" appearance when they molted, because head hairs molt before the rest of the body.
Mice molt multiple times throughout their lives and depending on what stage they are in, some coats are particularly apt to changes in color, especially brinldle and siamese/himalayan. This is very, VERY common in poor brindles. And if the mouse is recessive yellow too, the normal fault of a sooty back can make it even worse. Too, if the mouse is Agouti on the A-locus, that can cause the appearance we've all seen.
As evidence, here is a mouse I had a few years ago who had the exact same pattern:
It turns out she was Avy/A b/b e/e. I discontinued her line entirely. Besides being of terrible type, the genes involved weren't worth it trying to separate them to me.
