In my book it is written that in order to minimize systematic error in Mohr method for argentometry it is a common procedure to conduct a blank titration with $ \mathrm{Ag^+} $ against a $\mathrm{CrO_{4}^{2-}}$ solution (with the same concentration of the original solution).
The book then suggests to subtract the $ \mathrm{Ag^+} $ volume needed to observe precipitation (in the blank solution) from the volume obtained as a result of the 'standard' titration.
I don't understand why I have to subtract that number, specifically I do understand that there is a systematic error but I don't get why it is the volume of $ \mathrm{Ag^+} $ obtained by the blank titration.