So, here's the thing about Amazon rankings. They're not a meritocracy, and you might think that good reviews help you and bad reviews hurt you. They do/can! But it's closer to SEO than that. Amazon ranks things by what sells, and yes, higher-ranked things sell better, but it's more complicated than just that. Not everybody finds Perl books by looking in the Perl category. I'm looking at the third book you highlighted, about PHP. Its sales rank is 38,xxx and yours is 2xx,xxx (btw, and you may know this - the sales required to move up rankings increase exponentially, so this difference in ranking translates to quite a few sales per day). As long as Amazon believes it's about Perl, even if only tangentially so, it will rank higher in Perl than you because it's selling better. It's in their interest to market what sells better for them.
I learned this from fiction, but it applies to nonfiction too. First of all, books are categorized by what you put in the keywords section in KDP, if they're self-published, or by whatever your publisher put in there if you're not. Let's say you write a book about Perl and cooking. It may outsell pure Perl books because more people like cooking than Perl. It might be a great cooking book but a poor Perl book - or maybe you're lying about the Perl thing, and it's just a cookbook. But it'll top the Perl category as long as Amazon believes you that it's about Perl, and among the Perl books, it will have the highest sales rank.
Unfortunately, what I bet is happening is that this book is legitimately somewhat about Perl, and the author tagged it as such in the keywords. But its sales aren't coming from winning the Perl category alone; they're coming from that and PHP and beginning programming and generally from being ranked in multiple categories.
Your best bet to get your category back is to try to convince Amazon it's not about Perl at all. Good luck - I didn't look close enough to see if that's a reasonable claim or not.
The books aren't about Perl; they're gaming the Amazon system. That being said, it was interesting when you commented about sales rank. Yeah, they sell more than I do, but you can frequently buy the for $0.99, when my sells for a real price. That might make them break even, but since they're (I'm assuming) self-published, they're probably making far more money overall.
I learned this from fiction, but it applies to nonfiction too. First of all, books are categorized by what you put in the keywords section in KDP, if they're self-published, or by whatever your publisher put in there if you're not. Let's say you write a book about Perl and cooking. It may outsell pure Perl books because more people like cooking than Perl. It might be a great cooking book but a poor Perl book - or maybe you're lying about the Perl thing, and it's just a cookbook. But it'll top the Perl category as long as Amazon believes you that it's about Perl, and among the Perl books, it will have the highest sales rank.
Unfortunately, what I bet is happening is that this book is legitimately somewhat about Perl, and the author tagged it as such in the keywords. But its sales aren't coming from winning the Perl category alone; they're coming from that and PHP and beginning programming and generally from being ranked in multiple categories.
Your best bet to get your category back is to try to convince Amazon it's not about Perl at all. Good luck - I didn't look close enough to see if that's a reasonable claim or not.