If I understand the situation correctly, that is expected is it not? If all your products (lets say bikes) have all red color and blue handles and 100$ price tag, their SOP will be the same, since there are no differentiating factors. 100% will be divided into number of products in your scenario and that will be their SOP.
If you only change price across these identical bikes, you will end up with bikes taking different SOP. Lets say :
The change in SOP is only due to change in price levels. Now as long as you keep all the other levels identical for each product, the only attribute affecting the SOP will still be price. Hence if you change all bikes to be green colored, their SOPs will stay the same.