how comput stride size of vgg outputs?

[sanhai77]

hi
in the 3.3 section :

On the re-scaled images, the total stride for both ZF and VGG nets on the last convolutional layer is 16 pixels, and thus is ∼10 pixels on a typical PASCAL image before resizing (∼500×375).

vgg have 4 pooling layers(used in faster r cnn), each which has stride 2. and finally output stride will be 2x2x2x2 = 16. but i dont understand how this comput for (500x375) image size, is 10 pixel?
Shouldn't the step be 16 for all images?