Sometimes lethals pop out of sows that don't have any roaning whatsoever. Or the mother may be roan, but you know who the father is and he doesn't look roan. There are basically two possibilities.
New mutation in roan gene: Your baby lethal just happened to be so unlucky that he got a deformed copy of the roan gene from one parent, and then when he should have gotten a normal copy from the other parent, somehow in the DNA copying process, there was an error and now that one doesn't work either. This is probably not that common, but mutations are known to happen.
Roaning is hidden in the parent(s): You might not have looked close enough to see that Dad has a spot on his back where five white hairs are mixed in with his 50 million black hairs. Or Mom has other genes causing white hair, and roaning has no visible effect. For example:
We know (or suspect, because it
could have been a new mutation that caused Einstein to be a lethal) Suzi is a roan/dalmatian. However, most of her hair is white because of other genes, specifically the spotting gene, which is the normal gene that causes white patches on guinea pigs. Even if her pigment cells are in the roan pattern, the ones that are there don't give her hair any color, so you can't see the characteristic mixed white and colored hairs that produce the roan pattern.
Breeders warn each other against breeding the roan gene into lines of guinea pigs with the spotted gene (most pigs with white spots, e.g. tortoiseshell and white, black and white, etc.) as this apparently often produces guinea pigs who are nearly all white with patches around the eyes and possibly on the butt. So if you get a pregnant sow who's mostly white with eyepatches, be on the lookout for lethal babies!
Himalayan and all-white pigs can also easily hide that they have a deformed copy of the roan gene and could produce lethal pups, since they're all or mostly white. Some scumbag breeders even take pride in producing "white roans" for whatever reason, even though they look no different from a normal white pig.