From: near Philadelphia
8.2.11 Division into Sub-Units (Advanced Rules)
Units may divide and recombine. This is done using the Divide / Recombine button in the Unit Report window (see 16.1), or from the popup menu you get by right clicking on the unit icon in the Map Panel. When you choose to divide a unit, it can be divided into either two or three sub-units at your discretion. In order to recombine into the original unit, all sub-units must be present. If any sub-unit is lost, the unit may not recombine until the sub-unit reappears as a reinforcement.
Sub-units will have only about 80% of the parent unit’s proficiency. If you recombine your units, the newly recombined parent unit will have 125% of the average sub-unit’s proficiency. This means that the act of dividing a unit and recombining it will result in no permanent loss of the unit’s Proficiency.
It could be the reduction in proficiency reduces the movement points available. Or like Lobster suggested, other units in the desired path may increase the movement cost to enter that hex.
War is not always predictable.
Avatar image was taken in hex 87,159 Vol 11 of
Vietnam Combat Operations by Stéphane MOUTIN LUYAT aka Boonierat.