Housing in DC is a little over 3x what it is in Baltimore (overall so I'm sure you could find exceptions). In addition to that, you don't work for the federal government to make a lot of money. Because you can't.
So yes the salaries are not comparable. You would make less even if you got Step 10 of GS-15 (which you definitely would not). You might be able to make on your W2 at USDS what you make in Baltimore. Maybe a little less. If you work for the government in DC you are doing so out of a sense of service.
It's part of why it's so hard to get good tech people to work for the government. You have to be a Matt Cutts and have made your money already, or come from money, or have that inner drive for public service. It's hard to make $115k in one of the most expensive cities in the country when you could live almost anywhere else and make more each month and pay less to live there.