Does every solute spend at least $t_M$ in the mobile phase? In 'Fundamentals of Analytical Chemistry' 9th edition by Skoog, West, Holler and Crouch, there is the following passage (page 865).


  The dead time (void time), $t_M$, is the time it takes for an unretained species to pass through a chromatographic column. All components spend at least this amount of time in the mobile phase.


Apologies if it seems a silly question, but how does that make sense? Wouldn't all components spend exactly that amount in the mobile phase? That is, of course, assuming a constant flow rate for the mobile phase. Is this where I have gone wrong?
Dead time ($t_{\bf M}$, also called holdup time) is the time it takes for the mobile phase (eluent) to traverse one length of the column, for a given flow rate. Because moieties in the eluent do not interact chemically with the stationary phase, $t_{\bf M}$ is therefore a lower bound on retention times for the species that do interact (chemically) with the stationary phase. 

Simply put, anything that does interact with the immobile phase cannot elute faster that the pure mobile phase: retention times will be greater than $t_{\bf M}$.