This is something you get with FFTs I'm afraid.  The FFT gives you an array of energy levels across the total frequency range (half the sampling rate or 0 - 22050 Hz in your case).  The number of cells in that array (ie. the number of frequency bins) depends on the buffer size that you use.  If you change the buffer size from 4096 to 512 you'll notice that the detected frequency changes.  Minim hides all this by pretending that it can query any frequency but actually it can only give you the level of the part of the spectrum rounded to the nearest bin.  If you increase the buffer size you'll get a more accurate reading but the cost of this is that the response time is slower (so if the frequency is changing rapidly it won't react as fast).