It's common for Bandit speedos (and most Suzukis from what I can tell) to read a higher than actual speed. The 2G B6/12 are usually off by 7-12%. Surprisingly, the odometers are usually VERY accurate (<1% error typically).
If your speedo is off the other way, I would strongly suspect the gearing is not stock if your B6 is a 2000 or newer. It's also possible a weird rear tire size could throw it off... but not 20%.
If your B6 is a 1999 or older, it's a mechanically driven speedo off the front tire. About the only thing here to alter the reading would be tire size... or maybe a speedo drive from some other bike that has a different ratio.