Carnival says the excursion has to be exactly the same. Many of Carnival's excursions serve ham sandwiches as part of the price, but the independent tour doesn't. So a 50 cent ham sandwich lets them get out of the price match. That's why Carnival says very few tours qualify for the price match. They have all these exclusions and gotchas.
If the independent tour won't pick you up at the pier, then it is not comparable because all of Carnival's tours do.
Don't shoot the messenger - we're just trying to save you time.