Negative Buffer
If your ratio is below 1.0 and you have uploaded less than the amount you have downloaded, your buffer will return a negative number. Before we illustrate this issue you will have to know the difference between buffer and ratio.
Ratio
Basically what you have is your uploaded amount split with what you have downloaded. As with any other math problem you'll have to use the same unit (tb, gb, mb, kb, b).
Upload: 50GB
Download: 15GB
Ratio: 50GB / 15GB = 3.333
Buffer: 50GB - 15GB = 35GB
Buffer
Buffer is simply how much you can download before your ratio reaches below 1.0. If we switch around above numbers so that it looks like the following - not only would you be so much banned that you would have travelled back in time to the stoneage, but your buffer would also be very negative.
Upload: 10GB
Download: 22GB
Ratio: 10GB / 22GB = 0.454
Buffer: 10GB - 22GB = -12GB = -12,228MB = -12,582,912(KB)