After descending from a high altitude watervapor will condense into liquid..after a long transpac it practically flows out of the roof onto the pax. I think this represents a gain, however small.

There is also probably a "consumption" of oil or a slow hydraulic leak, as well as a loss of rubber from landing

There may also be slight gravitional variations on different parts of the planet that are miniscule but still play a tiny factor. Additionally, as the aircraft is farther from the surface of the earth while flying, it should have a teeny tiny drop in weight at cruise which it gains back on the ground.