Sometimes, but not always. Example: If I put a bowl of Grapenuts and Kroger brand Nutty Nuggets (hey, I didn't name it!), I'll bet 95% of the people could not taste the difference.
I can usually taste a difference between name brands and no-frills stuff; you get what you pay for. Some things are easier to substitute than others.
I’m surprised anyone can sell a more-expensive name brand product if they are so similar.