Depends on the router. For example some older, single antenna systems, use b. g and n on the same signal and drop back to the lowest common denominator when there is a number of devices connected.
Other things that affect it include neighbours transmitting their WiFi on the same channel, the construction of the wall despite it only being one, the software drivers for the WiFi card in the laptop not quite being fully compatible with the standard or other RF noise, e.g someone close by using wireless video senders.