Sky as the Light source.
On a clear sunny day, one can make out the direction in which the sun is shinning but on an overcast day the sky itself becomes a light source and a clear point light source, the sun is not visible. so even though you may not actually be shooting against the light , thats exactly what you are doing as the soft diffused light is coming from all over the over cast sky.
Canon eos 5d mark II 97mm 1/100 at f8 partial metering
canon eos 5d mark II 70 mm 1/50 at f8 partial metering
The difference in the above two shots is, that in the first photograph the sky occupies about 70 % of the space and the metering is influenced by the overcast sky. As the frame has been zoomed out in the photograph bellow, the area of the sky is relatively less compared to the area of the buildings in the photograph, hence less influence from the over cast (light source) sky.