The relative potential error in an estimate formed by the difference of two estimates of the same sign and roughly comparable magnitude is frequently very great.

Estimate of a is 1000 with a potential relative error of 1% so $990 \le a \le 1010.$

Estimate of b is 900 with a potential relative error of 1% so $891 \le b \le 909.$

So estimate of a - b is $1000 - 900 = 100.$ But

$990 -909 \le a -b \le 1010 - 891 \implies 81 \le a - b \le 119.$

The potential relative error is 19%. Once you understand this, you will never read financial statements the same way and will have a clue why accountants are obsessive.