raagamuffin as I'd mentioned, dealer prep can mean all the difference in the world to a piano's performance. Maybe it doesn't matter when you're a beginner but a dealer could sink thousands of dollars of labor into finely prepping a piano to be its best.
There is also after sale service. Some dealers just do no prep and make the sale and that's all they want to do, so maybe they offer bargain prices. Other dealers could put a lot of work into their pianos and also if you have any issues you can expect them to take care of you, and they have to build that into their pricing.
And then there's the size of the dealership and where they're located (rent costs; ability to sell at a lower profit margin and still survive) and also some pianos have been sitting on the floor for some time and maybe they're motivated to move it and are willing to offer a better price. Pianos are big instruments and require regular tuning even if they're just sitting there, because of weather/humidity changes. That's an ongoing cost for dealers who want to show their pianos in tune!
Even if you're looking at the same brand and model of piano, at two different dealers they could sound and perform like two different instruments depending on the level of prep. And pianos are unique, too, they are made of wood and felt and each one can be a little different (although some brands are known for their consistency more than others).
I know you're approaching this like buying a car or a digital piano where it doesn't matter where you buy it from, a new car of the same make and model should be the same. But that's just not true for pianos, and once you get past the beginner stage I always advise to get a piano that you love and feel a connection with. An unprepped, out of tune piano may be the cheapest but it's definitely not going to be that instrument for me. Hopefully this helps you understand why the final price is going to be a range, by necessity 🙂