This is correct. I was doing well in my career before the book, but it really took off after the book. However, I was already co-author of one book before that, and I blog prolifically, so that skews this.
The way it works is simple. Publishers hope to sell about 5,000 or so copies of your typical tech book. That's enough to keep the publisher going, but not enough to really make money for them. The publisher keeps those "filler" books going to keep up street cred and hopes to sometimes land the "killer" book that makes them a nice profit.
I can't share the sales info for my book, but let's just say that I think the book is doing better than one might expect for its category.
If you want to write a tech book, you want to go with O'Reillly. My advance was much larger than what O'Reilly would offer, but they sell more tech books in the long run. However, O'Reilly's Perl book section is saturated and they generally don't take new authors.
What I did to reposition my book was a strategy I wish more tech books would take: I focused on jobs. Instead of teaching every esoteric niche of a language (though I covered it well), I focused very heavily on skills that employers look for. I've been doing this for a long, long time and am very familiar with employer needs, so I was well-positioned to do this. That has led to my book doing better than expected: here's what you need for a job!
I now have a successful Perl consulting firm (http://www.allaroundtheworld.fr/) and that book, while I can't prove it helped, definitely didn't hurt.
Oh, and my publisher has spoken to an Amazon rep and a contact I have in Amazon tells me that they're aware of the situation and are looking into it. I don't have much hope, but who knows?