Hi gals / guys!

I have query relating to WiFi, its like this suppose there are more than one AP's in a Wireless network with same SSID and there are devices like Laptop, PDA's and other hand held devices connecting to them. Its been observed if device 1 is connected to AP1 and while roaming device 1 get close to AP2 but still its connected to AP1 although the distance b/w Device 1 and AP2 is much small than that with AP1. WHY is this so?

Some one told me that it will automatically connect to AP2 once the minimum signal strength value will reach. So if this is the case than what is that minimum value and can that be defined manually in a wifi enabled device.