When a piano has not been regularly maintained, it often drifts very far from its target pitch center. Most pianos are manufactured to perform optimally at A440: with the note A4 being tuned to 440hz and everything else tuned to that.
When a piano has been neglected for a number of years, it’s not uncommon for it to be as much as 100-200 cents flat – 100 cents being the distance between two notes on a piano. In other words, a piano that is 100 cents flat sounds a B when you play an C. This is a problem for many practical reasons, but the most basic is that the piano is designed to work and sound best at a certain level of tension on the strings, and anything significantly less is compromising the performance of the instrument.
The other fact that many piano owners do not realize is that pianos do not go uniformly out of tune. As you can see in the picture, the notes on this neglected piano vary from 90 cents flat to 176 cents flat, just within a couple of octaves. That’s the difference of almost an entire note! Obviously, tuning the piano at a pitch lower than A440 (known as “floating” the pitch) is not a good option, since it would still require massive adjustment of some notes for the piano to be in tune with itself. And large variances like this are quite common in instruments that have not been regularly tuned.
Keep on top of your piano tuning! You don’t want to be in the position this piano is in, because the piano will take multiple tunings to be stable again, and the longevity of the instrument will be negatively affected.