Hi Fellspade5390, thank you for the question. When expressing the fractional change $1.898 \times 10^{-15}$ as a percent it's necessary to multiply by 100 since "percent" means divide by 100. In other words percent means the number is expressed as a fraction where the denominator is 100, but the convention is to not write that denominator and write only the percent sign, %, instead. In order for the quantity to remain unchanged after "dividing by 100" (which the percent sign effectively does) it is necessary to multiply the number by 100, hence $10^{-15} \times 100 = \times 10^{-13}$.
Hope this helps,
Shaun