Or reporting it differently.
That would be my guess too. "Signal strength" is not determined by software, but the electrical characteristics of the transmission and receiving of the RF signal. Unless you changed the power supply or motherboard too, the RF signal should be the same. So that leaves antenna orientations, obstacles, interference and distance. Assuming you did not change the AP's or adapter's antenna orientations, add a wall or two between the two, or move the computer further away, the strength would remain the same.
That leave interference - which can come from many outside sources (other networks, microwaves, etc.) - or, again, the reporting.
Can you provide specifics?