I observed it for nearly 4 months. Minecon's cloak has increased unexplained reasons. The abnormal increase in 2011 2012 2013 2015 2016 is a safety issue. There are two picture comparisons Already
Already inquired Mojang's cape One of https://help.mojang.com/customer/en/portal/articles/872983-minecon-capes
The star tag in the lower right corner of the two images represents the account holding the cape.
The first photo was taken at 2019 3/31
The second picture with poor quality is 2019 7/12
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Name MC only knows the accounts you give it. When an inactive player types in their name for the first time or joins a public server prompting someone else to do it than it is added to the list of known capes. No specific api call exists that will return a list of all the capes in existence.
Another possible cause is those who never have the cape equipped will not show with a cape either. However once you equip it and lookup your name, Name MC will than forever cache it.
The last and only reason which is much more unlikely is the one stated above. Surprisingly some never redeem their code.
Talking about the capes that are not from old events like Minecon I can say that I do know how to get most of the oter capes because of a YouTube video. Because of that I know how to get the translator cape, the Moderator Mojira (For Moderators right here, in this site, like the guy who commented to you) and some more... I think it is the main reason, YouTube is really powerful in leaking information about everything.
It is possible for users to redeem their capes any time after the event, so it's possible the numbers reflect this too.