That's because your signature is TOO large and therefore is being resized automatically. We have limits on how big signatures are (otherwise people would abuse it). If you no longer want to see the yellow band you need to resize your signature per BP.net standards.
5. Signatures must be limited in scope. No more than the equivalent of 10 lines of standard text in a 1024x768 resolution. (A size 1 font will allow for more lines of text.) Banner graphics must be limited to a maximum of 450 pixels wide by 100 pixels high and 150 kilobytes in size. Members sporting extraordinarily flashy graphics in their signatures may be asked to remove/change them. Signatures including both graphics and text must still not exceed the equivalent of 10 lines of text.