This has long been a bug with Linksys firmware across various models. Even my old WRT54G had this bug. Basically, it doesn't seem to correctly identify which interface devices are using fairly often (WiFi vs LAN).
I suspect that the dhcp server considers the WiFi and wired interfaces to be part of the same LAN and doesn't really care which type of LAN interface a client is connected to. I have encountered this in other vendor's routers also.