For the percent function the result is correct: 9 -10% = 9 - (9 * 10/100) = 9 - 0.9 = 8.1. I don’t recall it ever working otherwise. I also don’t see how it would work, given pocket, PoS, and office calculators do tail operations only.
However, the warning holds in general. Given you emphasized the part about reviews: beyond calculators. Don’t trust those review systems. If anything, read only negative reviews and consider them only after carefully reading and evaluating.
Even if the reviews were honest, and often they are not, the entire system can’t work. Consider, who posts those opinions. Those are completely random, anonymous people, under no obligation to express anything beyond their first emotional impression, in overwhelming majority lacking expertise to provide a reliable opinion and more often not having basic skills to conduct a test, and with the comment window too short to allow for a proper test anyway. I’ll also skip the entire topic of who runs those review systems.
If you want an opinion, there is a lot of people doing honest, proper reviews. Dave is one of them. You know, who gives the opinion. You can judge their abilities, their knowledge and experience. You know their testing methods. Even somewhat entertainment(1) YouTube channels, like Project Farm, are worth orders more than any random opinions system.
(1) I do respect the channel and that comment isn’t meant to belittle it. He does a great job. But some limitations, like being able to afford only a single specimen being tested, hamper the strength of the results. Some tests are also humorous and I believe this is intentional. Nothing wrong with that as long as one keeps that in mind. To be honest it would be detrimental to Project Farm, if tests were proper: it would then become a horrible, boring job.