In a multi-rate wireless network, stations maintain a low error rate by adapting their transmission rates according to the channel quality, e.g. reducing their rate as they move away from the base station. This creates variations in channel demand as the duration required to transmit an amount of data depends on the rate used, which continuously changes due to mobility. The aggregate channel demand is determined by the spatial distribution of the stations and their mobility behaviour. Previous research showed that the Random Waypoint Mobility model could concentrate stations near the centre of the mobility area. This paper presents an analytical model to study the implications of this effect on the achievable throughput of a cell and the variability in the channel demand. It was found that the throughput could be over-estimated if the cell coverage was comparable in size to the mobility region, but this effect diminishes as the mobility region becomes relatively larger. We illustrate this effect quantitatively with IEEE 802.11 WLANs as an example.