Once you have those figures, it is deterministic; however, at the start of the month, those figures are not yet determined. One might win a small prize in a lottery; the price of some staple might unexpectedly increase or decrease; an aunt may or may not send an expensive gift; a minor traffic accident may or may not happen, requiring immediate expensive repairs.
So there are factors that you cannot control that affect your finances.
Once you have those figures, it is deterministic; however, at the start of the month, those figures are not yet determined. One might win a small prize in a lottery; the price of some staple might unexpectedly increase or decrease; an aunt may or may not send an expensive gift; a minor traffic accident may or may not happen, requiring immediate expensive repairs.
So there are factors that you cannot control that affect your finances.