Dear mechanical engineers, explain how no damage occurs for x number of miles, but miraculously, under the exact same conditions, suddenly after a certain number of miles, damage begins happening.
If I had to posit a guess, I would say that either increased wear is beginning immediately, but the cumulative amount of wear doesn't affect performance until after a some duration. Else it's still enough oil to prevent increased wear, (i.e., close to the minimum required amount of oil, and you could go hundreds of miles without damage, provided the oil level doesn't decrease further.) I.e., the designers realize that if it starts getting low that it's more likely that the cause is going to cause it to continue to lose oil than the cause was an oil thief stealing a quart. Get oil soon, before it continues to get low. The rate at which it is losing oil is incredibly important to make any determination as to how far you can go.
-trained as an engineer, but not employed as one.