Piano out of tune after tuning

After tuning, why many client reply there are out of tune? Why the sounds heard unstable, seperate and divergent? Like economics system, there could be two main reasons.

Unsystematic factor is privilege.

Piano have cast iron plat, soundboard and Pinlock. The original structure bear the total tension of around 230 strings. Total stretch force could be maximium 20 tons for one piano. Stability could be challenged by the materials and qualities of these internal parts. From intrinsic, they are very vulnerable from outside enviorment ,like Beta (β) measure of the volatility of outside,like temperature, humidity, moving or packing. Even outside is not changed, those parts could be unstable after tune. Systematic factor could be important too

Intonation，timbre and feeling could be changed if the outside condition changed. Grand piano used in concert, tuner sometimes need to tune the piano for massive use in the break of concert. Many musicianes practice everyday with big load. Every strings could vibrate and make internals parts bears stress and pressures. Piano relative humiditybetter range between 45% and 70% and a constant temperature of approximately 20˚C.

Better piano tuner they will tuner the piano for twice, because they want to put the 88 strings adapt tension for the optimal intonation for the best sounding effect.