What advantage would there be to not include the Bonus/acre in the Lease that is recorded at the courthouse?  I've seen this in a few leases where it states something to the effect of "in consideration of $1 and other valuable consideration."  It seems that the Lessor would want this in writing as part of the lease so that if a Lessee doesn't produce within the primary term, the Bonus +10% is clearly defined for the secondary term.

It may be some way to keep the Bonus amount from being public information and not getting published in the local newspapers....not sure though.