A mathematical answer is presented for the frequent occurrence of false or "fictitious" Rb-Sr isochrons. The reason for these inconsistencies is that a simple linear regression procedure is mathematically invalid if two or more independent variables influence a single dependent variable. In many data sets for the "isochron" procedure, there are two independent variables involved. First, there is the desired radioactive relation between the amount of the rubidium parent and the strontium daughter. Second, since the atomic strontium concentration in the samples is a variable, then the isotopic Sr-87 content of the atom is also a variable. In such a situation, the "Isochron" regression is mathematically invalid, so both its slope and intercept are erroneous.