Why does a difference in potential of 4.9 volts cause the electrons to move with an energy of 4.5 eV instead of 4.9?

My textbook, when talking about the Franck-Hertz experiment, says that the intensity of the current starts going down when the difference in potential reaches 4.9 V (or more specifically 4.86 V), because at this value the emitted electrons reach an energy of 4.5 eV which is the threshold for the mercury atoms to absorb the energy. But wouldn’t a U = 4.9 V cause the electrons to have an energy of 4.9 eV (instead of 4.5), since an eV is defined as the work done to an electron when it moves through a potential difference of one volt?

Edit: after some more investigation, I think that the value in the book is wrong, and both values should be 4.9, though some confirmation would be nice.