The difference is due to the different methodologies: Moody's uses its own growth projections while S&P defers to the city's growth assumptions.
We have blogged extensively about the systematic underfunding of municipal pensions and the political pressure to under-save:
The city of Nashville uses discounting to decide how much to save for its future pension obligations. For a pension that pays out $100,000 in 20 years, Nashville must save $20,485=$100,000/(1.0825)^20 today, using an 8.25% discount rate. If the city invests the $20,485, and earns 8.25%, the savings will compound and be worth $100,000 in twenty years. If however, the investments return less than 8.25% (in fact they have done much worse),then the city will not have saved enough when the future finally gets here. Of course, a more realistic discount rate, say 6.5%, would mean much higher current savings, 28,380=$100,000/(1.065)^20 to fund the same future pension. But higher savings means less current spending, and spending is politically popular.
1. City assumptions are typically too high and are almost always backward looking, so after a long period of high returns, the assumptions will be for continued high returns. When in fact, high returns in the past are correlated with lower returns in the future.
2. I suspect that S&P gains clients for its consulting services from many of the same cities that it rates. Its favorable ratings may be an advertisement to financially strapped cities that S&P will come up with a low savings rate which will provide cover to politicians who want to continue their irresponsible behavior.
If this is correct, financially healthy cities may want to "signal" their financial health by hiring Moody's.