Muzzle blast can also give low readings. That occurs when the gas trips the front screen (before the bullet gets there) but not the back screen (so the timer has to wait for the bullet to get there), making it seem like the bullet was between the screens for a longer period of time than it actually was there.
High readings occur when the muzzle blast is strong enough and the screens close enough that both trip on the gas and never see the bullet, because close to the muzzle, the gas is travelling faster than the bullet. Really severe muzzle blast apparently can actually increase the bullet velocity a little bit AFTER the bullet has left the barrel, as the gas blows by the bullet. (But, the turbulence of that process can also deteriorate accuracy.)
SO, if you start-out setting-up your chronograph much too close to your muzzle, and then move it back in stages, you may see the muzzle velocity reading be way too high first, then become way too low, then finally be correct when you get far enough away.