This article is from the Toyota Tundra FAQ, by Steve Yee with numerous contributions by others.
This has bugged some Tundra owners, and has been reported as a problem by some.
The cruise control and the onboard computer sometimes do not agree, making or
forcing the cruise control subsystem to make wierd decisions like accelerating
without downshifting, or speeding up about 3-5 MPH above the desired speed,
then slowing down. This seems to be normal, but not desired, behavior from the
cruise control and the vehicle's computer.