The Confusion here is total attendance of the con vs unique memberships.
Otakon in 2018 had 28,116 unique members, 759 Staff, 294 Artist Alley Exhibitors, and 110 Dealers room exhibitors who may or may not be counted as members and may have multiple people per booth, and 38 Guests.
I'm assuming to get technical 'full attendance', Otakon probably added the staff and then some of the artist and dealers or maybe guests? and then came up with the 29,293 figure