That makes sense.. but immediately afterward both Harry and McGonnagal thought it was unusual how bright the remembrall shined; neither seemed to think it was solely due to the use of the Time Turner:

"The Remembrall was glowing bright red in his hand, blazing like a miniature sun that cast shadows on the ground in broad daylight."

and

""More importantly, why did the Remembrall go off like that?" Harry said. "Does it mean I've been Obliviated?"

"That puzzles me as well," Professor McGonagall said slowly. "If it were that simple, I would think that the courts would use Remembralls, and they do not. I shall look into it, Mr. Potter.""

And then of course she doesn't. Perhaps the courts simply don't use Remembralls because they would never definitively prove obliviation - only that something was forgotten. Harry's remembrall "blazing like a miniature sun" may be due to an overwhelmingly large obliviation - like an entire life as a Dark Lord? Obliviating a day or week may just produce a normal glow...