Why total mined sum differs so much on scan.idena.io website from one epoch to another?

Example:
for epoch #0011 (10/08/19 - 10/12/19, 4 days) total mined is 363,101.98 DNA
for epoch #0013 (10/16/19 - 10/20/19, 4 days) total mined is 247,992.00 DNA

Other epochs also differ significantly from each other. Why is that? I thought that this sum is easy to calculate for any epoch of 4 days:
51840 x 4 = 207,360.00 DNA.