Are you looking at a specific model laptop, or a range of configurations?
Often in a line they can use a higher power (8700m) or lower power (8500m) GPU in the same chassis - so for the range of configurations they will say it has an 8500m/8700m (if those are the two options).
Alternatively AMD has blurred the lines with their integrated GPU's working in crossfire with a discrete GPU - so the 8500m could be the integrated graphics only and 8700m performance level when using both... but this is just a stab in the dark. I do not know what models/numbers/etc can do this.