In effect it does. If you have read 20 pages and found no typos you estimate the chance of an unknown page having a typo is 1 in 20/3. But if there is 0 unread

In effect it does. If you have read 20 pages and found no typos you estimate the chance of an unknown page having a typo is 1 in 20/3. But if there is 0 unread pages then there 0 cases to apply the probability to. Make it 100 pages, then you have 100 cases each with a 1 in 20/3 probability. Of course if you read 30 more and still don't find a typo, it's now 50 remaining cases each with a 1 in 50/3 (estimated) probability.