Suppose you have 50 unseen cards, and you have it set to show you 5 new cards every day, and energy for about 10 reviews.
Then on day 1 it will want to show you cards 1-5; day two it will show you 1-5 again and also 6-10; day three it will show you 6-10 again and also 7-15 (to simplify somewhat). So you're having some cards you know and some that are new.
If you have 50 cards you've forgotten, then on day 1 it will show you 1-10; on day 2 it will show you 11-20; day 3 it will show you 21-30, and so on -- all completely new. That makes it far more of a grind.
Then on day 1 it will want to show you cards 1-5; day two it will show you 1-5 again and also 6-10; day three it will show you 6-10 again and also 7-15 (to simplify somewhat). So you're having some cards you know and some that are new.
If you have 50 cards you've forgotten, then on day 1 it will show you 1-10; on day 2 it will show you 11-20; day 3 it will show you 21-30, and so on -- all completely new. That makes it far more of a grind.