But not exclusively.

91 is part of the mayans calendar system.

"Every facet of the calendar system relates to the following numbers 13, 26, 52, 91, 104, 364, 891 and 1508. The author has shown in other papers that these numbers are part of a series and all are closely related"
[link to www.world-mysteries.com]

The calendar was based upon “synthetic numbers” that had THIRTEEN as the seed number.

And as we've demonstrated and will continue to do so 13 is key to the message of all the true monuments in relation to Orion's halfway point 26,000 yr transit.