Amazon is a big company that can pay big salaries to its decision makers so I'm pretty confident that there are smart people behind these decisions. Your maths is however useful because it does beg the question about what is going on. A couple of theories of mine are...
1 - For various reasons Amazon might get additional value from boosting it's Amazon Prime subscriber numbers. Perhaps that helps it negotiate better prices for licensing of externally made Prime Video and maybe other content such as other channels under the Prime umbrella (where it presumably takes margin on those subs). It also perhaps makes it more likely that any Alexa+ queries that result in a product recommendation could immediately translate into a purchase via Amazon. Even without a Prime membership Alexa+ could still say in an answer that some item the user needs is available from Amazon but it is probably a more compelling hook if Alexa+ can say "... available for delivery tomorrow at <some-price>. Do you want me to order it now?". This pricing difference you pointed out could clearly boost Prime membership numbers since for anyone wanting Alexa+ it is cheaper to sign up to Prime than take a stand-alone Alexa+ subscription.
And the more sinister theory...
2 - We're about to see an increase in Amazon Prime pricing!
