The cost adjustments you're talking about are MS-DRG adjustments (the amount the hospital actually gets paid by CMS). From what I can gather, the numbers on the website, are cost numbers, i.e. what the hospital tells CMS it costs them (which is a number they are free to make whatever).
If you want to know why costs vary so much, starting looking at reimbursement. Hospitals are incentivized to raise their list prices because many of them are paid a set percentage.
I thought medicare reimbursement is relatively fixed, with minor fluctuations based on geography and some hospital-specific practices? Based on a rough glance at the data, it seems like medicare reimbursement is only loosely correlated with the cost numbers, so I don't know how much raising prices would increase their Medicare revenue.
Also, from what I've learned about elderly care, Medicare reimburses a care center a fixed amount ($5000/mon/patient in California) regardless of the severity of the case. I'm assuming this number varies a bit too even in the same region.
As a side note, if you click on any of the prices in the map, it'll show you medicare's reimbursement rates too. Should've made that feature more clear--it's rather important.
If you want to know why costs vary so much, starting looking at reimbursement. Hospitals are incentivized to raise their list prices because many of them are paid a set percentage.