If the centreline is extended the VNAV path is calculated based on the straight line distance from the aircraft's present position to the next waypoint. The VNAV path is effectively an angle calculated from your height above the threshold and distance to the threshold. If the path angle is the same as a the glideslope then they'll be coincident. Next time you're flying look at FPA and
VB. FPA is the gradient you're currently descending at and
VB is the angle derived from current height above threshold and current FMC calculated distance to go.
Picture it this way: if you're on a right base and the centre fix point "CF09" is to your right and you've extended the centreline by direct intercept CF09 with an inbound course of 090 then the VNAV calculation determines the distance to go as a straight line to CF09 followed by the LNAV distance CF09 to the threshold. Hence, if it's a wide base and long final and CF09 is only a few miles out from the threshold then VNAV will be showing high when you're not (due to your actual track miles being longer than that calculated by the FMC based on your inputs).