Doppler shift equation results in inconsistencies

The non-Relativistic Doppler shift equation is $f = \left( \frac{c + v_\text{r}}{c + v_\text{s}} \right) f_0$ where c is the speed of the medium (346.4 m/s for sound at 25 C temperature). I tried calculating the Doppler shift for the case when the source was moving towards the observer, and the case where the observer is moving with the same velocity towards the source, but I get different answers by about 0.75 m/s. Why does the frame I choose make a difference?

http://www.wolframalpha.com/input/?i=200*%28346.4+%2B+20%29%2F%28346.4+-+0%29

I am running the calculation for a wave with 200Hz frequency and a source(or an observer) moving with 20 m/s. $v_s$ is velocity of the source and $v_r$ is the velocity of the observer.