Strange connection behavior re SSDI

Connecting to municipal wifi with no encryption.
Using XP and a usb dongle. Most of the connections to the
same MAC address do not list an SSDI. I can connect without
listing an SSDI, but sometimes I cannot, sometimes I have to
enter the same SSDI as the AP Router in my usb software in
order to connect. Other times it will not allow me to connect
with the same SSDI and I have to erase it to connect. The
times when I have to enter the same SSDI as the AP are those
when other interfering routers are associating with my radio.
I have to enter the AP SSDI to get rid of them and associate
with the proper MAC address. Can anyone explain to me what is
likely happening?