Why he groaned before knowing what it was?

In the example:
I at first wondered why Ron groaned, not knowing what it was in the parcel. But we can see he’d already seen what it was, now that the next sentence has the perfect tense. Is this right?

"I think I know who that one's from," said Ron, turning a bit pink and pointing to a very lumpy parcel. "My mom. I told her you didn't expect any presents and -- oh, no," he groaned, "she's made you a Weasley sweater."
Harry had torn open the parcel to find a thick, hand-knitted sweater in emerald green and a large box of homemade fudge.