Single strung tuning stability

Greetings,

Having waited 13 months since the last tuning my piano was still rather well tuned. The unisons were all fine, just the high treble and the lowest note had dropped a bit. Not wanting let anything get too bad, I called the tuner. He was not surprised to find it to be still well tuned after a year, saying that Bluethner is known for it's stability. Has anyone else heard this? Here are some possible explanations I thought of. Soundboard ribs machined to maintain crown; massive, stable bracing; single stringing; very old and settled in; Room humidifier maintaining RH above 40%, (although lacking a dehumidifier this summer, the RH often went well above 60%); played about 2hrs a day.

Thanks,

My guess is that the biggest factor in your case is age. It would seem that all the stretching and compressing of all the stressed parts will have been stabilized long ago. TS