It is probably the case that Joseph is not yet ready to allow only one of the brothers to leave, because he knows their families are suffering from the famine, and one man cannot take enough food to save them. As verses 18-20 show, he therefore modifies his plan to test them. However, in the meanwhile he puts them all in prison.

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .