The concept of "roaming", i.e. moving among access points, was not high on the priority list for the framers of the 802.11 series of specifications. As a result, roaming performance varies widely from product to product and for SOHO-grade equipment tends toward the terrible.
Roaming logic is built into wireless client software (and sometimes firmware) and tends to be set so that clients hang on to the signal they have until the AP signal pretty much completely disappears. We've occasionally seen a wireless client that has allows a selection of optimizing for range or speed, but the control is usually buried deep in the network properties.
"Enterprise" grade products, especially entire WLAN management / security systems tend to have faster roaming switchover times, but much higher prices.