The former is likely to increase the rate of initial colonization and subsequent spread and the latter is likely to compensate for a low rate of initial colonization. Its all a matter of space for roots and space for the fungi to colonize those roots. This recommendation was made to accommodate a wide range of host species. Four months is not any sort of magical time period. Maximum investment in vegetative growth occurs when roots are most actively growing.
Minimum investment occurs when root growth has slowed or ceased.
Therefore, sporulation gears up tremendously after the host plants become root-bound. Sporulation, if it occurs at all, likely increases only in roots that have ceased growth and host carbon and nutrients are reallocated from mycorrhizal structures to spores. In pots, root growth becomes constrained relatively quickly and all fungi that have established sufficient biomass will then have host resources to sporulate. In most cases, I think its a juggling act between rates of growth by each symbiont and source-sink relationships at each point in time. However, the results obtained often do not match those involving whole soil or root inocula discussed in previous questions.