There are a number of reasons that could explain why supermarkets can sell their own products at such a reduced price in comparison to named brands.
Firstly, supermarkets are manufacturing the products for themselves, and so don’t need to pay for distribution costs, advertisements or for external labelling.
In addition to this they can manufacture the products in large quantities not having the worry that there will be nowhere to sell them.
Finally, brands such as Heinz, can afford to have a higher profit margin as they know there will be consumers willing to pay the extra for their brand and their unique taste. In comparison this makes supermarket brands seem even cheaper.