It probably has something to do with saturation.
Everyone recognizes the google adverts as google adwords. In other words they have been seen so much that surfers recognize them as adverts and ignore them more often.

Cross Timbers Royalty Trust is an express trust created pursuant to the Cross Timbers Royalty Trust Indenture entered in 1991, between predecessors of XTO Energy Inc. as grantors, and NCNB Texas National Bank, as trustee. The net profits interests are the principal asset of the trust.