I do not understand the sentence "Dividing 55,752 by 137,586 gives us a Day 3 retention of 63.8%," presented as the formula for overall retention in the document <How the Retention Analysis chart calculates retention> provided by Amplitude. Dividing 55,752 by 137,586 results in 40.5%, but it is stated as 63.8%. It seems that the denominator for the retention calculation varies for each day. If so, what is the reason for the varying denominator values in the overall retention for N-day retention?