Why do I often get Packets where the first high-order byte for the raw wave data is 0xFF (11111111)?

Why do I often get Packets where the first high-order byte for the raw wave data is 0xFF (11111111)? This is odd and it causes frequent spiking. This is clear in my serial data stream, but it doesn't appear to always show up using the BrainWave Visualizer. Am I simply reading the packet incorrectly? Does that 0xFF server a different purpose? Does the BrainWave Visualizer handling these spikes in a certain way?

The high-order and low-order byte of the raw wave value together form a 16-bit signed two's-compliment number. When the high-order byte is 0xFF, then the raw wave value is a negative number.

In C, you can get the number as simply:

short rawValue = (hiByte<<8) | loByte;

In other languages, you can try:

rawValue = hiByte*256 + loByte;
if( rawValue > 2^15) {
    rawValue = rawValue - 2^16;
}