Digital audio is by nature, a quantized format.  When a converter reconstructs old-fashioned sound from a digital signal, it basically is doing a lot of math to figure out what shape of a wave is defined by these digital numbers.  Sometimes, even if the numbers themselves don’t exceed a certain maximum, the mathematical curve the define could loop up past it.  This is known as “intersample clipping” – namely, clipping that happens to your signal between the individual sampling points.  It’s generally not a big deal, but it can add some audible harshness to your signal.  The easiest way to avoid it is to not peg your meter at 0db – these intersample peaks are rarely much higher than the samples around them, so adding a bit of headroom basically keeps this from happening.