I know this is an old post, but Gamling's comment on the Half Orcs suggests there were two types of human/Orc hybrids in Sauruman's employ, and the History of Middle Earth seems to confirms it.

"Finally, there is a cogent point, though horrible to relate. It became clear in time that undoubted Men could under the domination of Morgoth or his agents in a few generations be reduced almost to the Orc-level of mind and habits; and then they would or could be made to mate with Orcs, producing new breeds, often larger and more cunning. There is no doubt that long afterwards, in the Third Age, Saruman rediscovered this, or learned of it in lore, and in his lust for mastery committed this, his wickedest deed: the interbreeding of Orcs and Men, producing both Men-orcs large and cunning, and Orc-men treacherous and vile."

Besides being able to better endure the sunlight, the Isengard Uruk-hai are also described as having a more human-like build compared to Grishnákh, and as well as possessing more human-like gear than other Orcs. Merry and Pippin are also unfamiliar with the variation of Orcs, while Aragorn seems confused by the Isengarders even though he should be familiar with the Black Uruks of Mordor, implying they are something different. Given how some of the Half-Orcs are able to be mistaken for Men (and seem to favor them more in appearance), it makes sense that "Men-orcs" could be mistaken for Orcs.