Align on sector boundaries is a function that splits the audio into 1/75th second blocks, which is equal to the size of a sector on a CD. This function should only be used for 16-bit/44.1 kHz audio. Those bytes are the amount of the wave being trimmed off one file and added to the next (or vice versa, I forget the implementation - I think you can even choose if you use the command line) to make the first file end on a multiple of 1/75th of a second.