What would be the effect of on the calibration factor if the water isn't stirred properly?
And if 100ml of water is spost to be added but 90ml is added instead?
Well, given your question, I can try to answer to that.
1. What happens if the solution, after having added water, isn't stirred correctly? Well, in this case we'll have parts of the solution in the flask more concentrated than the average and some part less concentrated. So, depending on which portion you analyse, you'll commit a very large casual error on your measure.
2. What happens if we add 90 ml instead of 100? Well, first of all you NEVER have to add 100 ml of water to fill a 100 ml flask, because the other reactants occupy some volume, and the water required is less than that. Anyway, if we don't reach the mark in the neck of the flask, we'll have a more concentrated solution, and every time we'll analyse that we'll commit a systematic error on the measure.