In my opinion
Saruman's
Uruk-hai (which term means ("[great] Orc-folk") were
great orcs, better trained to endure the sunlight.
"But we can't run in the sunlight."
"You'll run with me behind you," said Ugluk, "Run! Or you'll never see your beloved holes again. By the White Hand! What's the use of sending out mountain-maggots on a trip, only half-trained. Run, curse you! Run while night lasts!"
"Behind them came two battalions of the fierce Uruks, heavily armed but trained to move at great speed for many miles." The
Battles of the Fords of Isen
Theodred was not slain by an
Uruk, but was
"hewn down by a great Orc-man."
And
Merry (Flotsam and Jetsam) describes the
armies of Saruman, including endless lines of marching
orcs, and troops of orcs on great
wolves,
and battalions of Men: "Most of them were ordinary Men, rather tall and dark-haired, and grim but not particularly evil-looking. But there were some others that were horrible: man-high but with goblin faces, sallow, leering, squint-eyed. Do you know, they reminded me at once of the Southerner at Bree; only he was not so obviously orc-like as most of these were."
Aragorn refers to these as
half-orcs, and notes that they had many to deal with at
Helm's Deep.
So with respect to sun endurance, I think we have well trained Uruks and Half-orcs (not fully orcs), and if we really want to press matters, back in the chapter the Uruk-hai, it's said that
most of the Northerners dashed off,
while a few of the larger, bolder Northerners remained with the Isengarders, and it's never noted that these larger Northerners have any trouble keeping up with Saruman's lot in the
race that follows.
In short, we appear to, at least arguably, have more than one reason why certain orcs or ork-ish beings do better under the sunlight than others, including the half-orcs who also might not quail under the sun.