Here you go...
The sea level presure in column A is 1000mb and the aircraft is flying at the 700mb pressure level.
The sea level pressure in column B is 900mb, so the 700mb pressure level is lower.
The aircraft will therefore descend with a constant altimeter reading because it is merely following the 700mb pressure level. Remember that the altimeter is simply a barometer.
All that has to be done in column B is to reset the subscale to 900mb.
So barometric error is not an error of the instrument. It may more aptly be described as "finger trouble" by flying with a mis-set subscale setting.
LM