Plant Experiment: Growth of a Geranium

[nbenvenuto]

For this Plant Experiement, we as a group planted two Geraniums One in Pot A and one in Pot B....over 19 days being watered in the same amount 100mL a day, we noticed that Pot A Plant grew larger in width and growth whereas, Pot B Plant did not grew that large....
The question i dont get is, what does he mean when he states, "ACCOUNT FOR the differences between the growth of each plant."

[mith]

If you treat them the same, then they should grow the same. Why not?