We show that if we assume the existence of a frame in vacuum where the one-way speed of light is c (c is the measured value of the two-way speed of light in vacuum) than for another frame moving with velocity v_1 in relation to that frame the speed of light in this another frame cannot be c. What is c in this another frame is the one-way Einstein speed of light. Although this has been known and affirmed several years ago, only recently we have an increase of publications stating the variability of the one-way speed of light. Therefore, since the constancy of the speed of light is sometimes considered an unquestionable postulate, we are assisting to an astonishing conflict between the "standard formulation of Relativity" and "Relativity with variable light speed". However, this conundrum can be easily resolved with the clarification of the conceptualization of simultaneity and synchronization.