In CUST-12, TE Frankie Hernandez accepted the largest offer ($33,826,654 over 6 years), but it wasn't the largest guaranteed offer. The guaranteed money is $11,543,856. I thought players signed the contract with the largest guaranteed money regardless of the total amount. I can't see the guaranteed money I offered, but if I do the math at 75% of the total offer, I should have been offering $13,000,000.

Is there something I'm not seeing??

The guaranteed money holds the greatest influence but the base salary does come into play although in a significantly less meaningful amount. Normally it's not enough to tip the scales but it may have been in this case.