For an oscilloscope, doesn't the 'bandwidth' specification mean that the frequency response is from DC to xxx MHz where the response will be 3dB down at the high end of the range ? In other words, the frequency range for your scope would be from zero to 100 MHz, but the displayed amplitude will be low at the higher end. You'll have to dig further to find out the frequency range where the drop-off starts (probably somewhere in the range of 60 to 80 MHz or so - a guess). Hopefully you'll hear from someone with a manual.