http://icsc.un.org/resources/sad/ss/sal201201.pdf
A senior developer is usually P-3 or P-4. It may seem on the low side compared to market rates if you in New York or Geneva - but bear in mind they don't need to pay taxes and get quite a lot of other benefits.
Edit:
This site has more details on the salaries, benefits and theory behind it:
Performance is how you move up GS and step levels.
There's a complex social dynamic related to that feature in companies.
https://en.wikipedia.org/wiki/General_Schedule_(US_civil_ser...
My rent in Houston was $1100/mo for a 2 bed, 2 bath 1200 sqft luxury apartment in a nice development with hiking and biking trails. My rent in NYC for a 1 bed, 1 bath, 600sqft luxury apartment in a nice building with a doorman every other day (idk why) is $2300. More than 4 times the price per sqft.
Every day staples are 20-100% higher in NYC than Houston. Car insurance is almost triple. Parking which used to be free and assigned is now in a garage and $200-400/mo depending on where you are (thankfully mine is in the rent).
I think the government needs to re-evaluate their adjustments.
Locality is to make a job's salary competitive with other job salaries in the local market, whereas COLA is exactly what it sounds like.
Houston has a high locality rate because of all the high-paying jobs there that one could potentially get instead of the govt job.
I work for the govt and receive both locality and COLA, albeit the COLA is way too low in my opinion (Honolulu).
In the WS scale the 3 highest paid is in KS, MI, and Atlanta. I think they consider what other positions in the same kind of fields are paying for the area.
[1] https://www.opm.gov/policy-data-oversight/pay-leave/salaries...