I see this all the time and I find it so confusing. My guess is that it has to do with an app being around for a long time and over time have offered upgrades.
This is the screenshot on the Mac App Store for 1Blocker. I have not tried to actually buy a yearly premium but as you can see, there are multiple options. My guess is that if I try to by the Premium 1 year for $4.99 it will know that I am a recent buyer and will only let me get the $14.99 option.