As the rug-addict mentioned about the numerous ways a floater can separate there is also length of runs which has limits and pinch points where the flooring is hitting a vertical obstruction, lack of a gap, heavy furniture, direct sunlight hitting the flooring and pivot points like at the bottom of stairs.
And you can still be out of flatness spec over ceramic even if its had the joints filled. Get a 6 foot level and run it around the floor in the affected areas and see if theres any teter - tottering going on. Check behind base boards or quarter round. Many installers who don't understand expansion will even caulk around some things like sliding glass doors and door casing. Caulking will inhibit expansion and cause gaps.