What is a pitch correction? In short, when a piano tuner comes over for an appointment, he or she can fine tune the piano in one tuning when the piano is close to pitch. If it's not close, than it takes two tunings to get it right. That first tuning is the pitch correction. Both tunings happen at the same visit. So why can't it be tuned accurately on the first tuning?
Your piano has over 230 strings stretching from the pin block down to a bridge and anchored onto the cast iron plate. The tension of the strings is applied directly onto the bridge in order to create the sound you hear when you play a note. But the bridge can be anywhere from three feet long and seven feet long.
When the piano is very flat, and the technician turns the tuning pin to adjusts the pitch, this new tension creates a new force of bearing across the bridge. Imagine a tight rope stretched between two trees with a little slack in the wire. You stand on the rope close to one tree and the rest of the wire tightens because of your force bearing down. Now imagine your friend stands on the wire close to the other tree. Their force (in this case, applied by weight and gravity) will add more tension to the wire, and it makes your body rise up a little.
So now we can envision the tight rope is the bridge of the piano, and when there is no one on it the pitch is 435. When the tuner tightens the bass notes to 441, that represents the first person standing on the wire. When the second person stands on the wire at 441, your pitch drops to ~440. So when the whole piano is tuned, its nearly impossible to accurately tune the whole instrument to 440, so they adjust the pitch to be a little sharp in one pass to get the piano roughly in tune. Once its at pitch, they can fine tune it and make it sound like a piano again.
Lastly, you may wonder why your piano needs to be tuned to A440. When the strings are all tightened to the right pitch, it creates around 20 tons of pressure bearing down on the soundboard. The soundboard of a piano isn't flat, it's slightly arched, or crowned, sort of like a contact lens. Pianos are engineered around the assumption that the pitch will be A440, and so they put more crown in the soundboard to settle at the correct shape when the strings are pulled tight. And when the pitch starts to go down, or flat, than there isn't the right amount of pressure against the board and the sound isn't generated in a way that is desirable. So your instrument should be brought up to the right pitch.
Also, if the piano is a teaching instrument, when it's flat, the student learns the incorrect sound from that piano and makes it difficult to practice on other pianos because the sound is very odd. When you want to sing with the piano, or play other instruments, it makes it very difficult to play together. The piano will sound flat unless all the other instruments, including the voice, are tuned down to the piano.