allowance for receivables in % and specific doubtful debt – net receivables

lecture 1 on irrecoverable debts example 1.

Intuitively,
why do we have to reduce receivables by the specific doubtful debt of $2,800 as well, (after irrecoverable debt has been deducted from receivables) before we calculate the allowance for receivables given by a percentage?

as in why do we have to deduct the specific doubtful debt on top of the percentage doubtful debt? doesn’t the percentage allowance sort of cover for the specific doubtful debt?

is it just a generalisation?

thank you.