We don't know the details of your setup, but most likely there was a partially reflective surface over the actual photoresistor. The photoresistor itself may also be partially reflective.
A higher fraction of the light is reflected off such surfaces at low angles. That means less of the light is transmitted to the photoresistor. Then the cosine of the incident angle applies in addition to the absorbent part of the photoresistor.
An easy way to see this reflective effect is to put a piece of ordinary glass between the light source and the photoresistor. Have the photoresistor directly face the light source. You will see less light hitting the photoresistor as you tilt the glass between the light source and the photoresistor.