In North Texas the air swings from damp spring weather to dry, heated winter air. The wooden soundboard rises and falls with that moisture, and because the strings are anchored to it, their tension changes and the pitch moves. This is why two tunings a year apart can look very different.
Many pianos go slightly sharp in humid months and flat in dry months. Tuning at a consistent time each year helps, because the piano tends to be in a similar state when the technician arrives.
Moving a piano, even across a room, can shift its tuning as it settles into a new spot with different light, temperature, and humidity. Older pianos also drift more as their parts wear. None of this is a flaw. It is simply how a wooden, string-tensioned instrument behaves.