Android Visualizer getWaveForm() behaves weird at high volumes

Question

I am using the Android Visualizer to obtain the waveform data of an audio stream and calculate the RMS of it.

public int[] getFormattedData(byte[] rawVizData) {
    for (int i = 0; i < formattedVizData.length; i++) {
        // convert from unsigned 8 bit to signed 16 bit
        int tmp = ((int) rawVizData[i] & 0xFF) - 128;
        formattedVizData[i] = tmp;
    }
    return formattedVizData;
}

//...

double rms = 0;
int[] formattedVizData = getFormattedData(buffer);
if (formattedVizData.length > 0) {
        for (int i = 0; i < formattedVizData.length; i++) {
            int val = formattedVizData[i];
            rms += val * val;
        }
        rms = Math.sqrt(rms / formattedVizData.length);
}

I set the capture size to 1024 and expected the RMS value of the signal to grow if the playback volume is higher. I test with a 440Hz sine test tone and get the following results:

     VOLUME        RMS  MIN VALUE  MAX VALUE  
      6.67%        5.5         -8          7
     13.33%       13.3        -19         18
     20.00%       28.1        -40         39
     26.67%       42.6        -60         59
     33.33%         68        -96         95
     40.00%         54        -77         76
     46.67%         81       -115        114
     53.33%         64        -91         90
     60.00%         51        -91         90
     66.67%         68        -97         96
     73.33%         48        -69         68
     80.00%         68        -97         96
     86.67%         45        -65         64
     93.33%         64        -91         90
    100.00%         90       -128        127

Why does my assumption not hold anymore for volumes > 33.33%? What am I missing here? Is there some obvious bug in my code I just seem to be unable to find?

I tried for days now finding out why this is not working and so far couldn't come up with a solution.

Any help is greatly appreciated.