My guess is that the figures are inaccurate as hell for total figures. Self-reported figures involving marketing are usually done for a reason, and you have to think through the reasoning.
I know a new car dealership that advertises that it makes over one million dollars in sales every month. Pretty big volume to move a million dollars worth of new cars a month, isn't it? Or is it? Actual breakdown of its sales is about $350k in new cars, $350k in used cars, and $300k in parts. And if sales are down in a month, a whole bunch of used cars are sent to auction to ramp up the figures. But by advertising over one million dollars in sales a month, it looks higher volume than it actually is.
People respond to this sort of stuff, and anyone in marketing knows it. Look at Powerball. To play one set of numbers for an entire year costs $104. So what is the average player win in a year? Based upon a 40% payout, the average player of Powerball wins -$60 a year. And yes, that's the negative in front of the $60. The average Powerball player would lose $60 per year, year in and year out.
So why do people play? Because they might win millions!
This is all basic psychology. Let's think about it from the point of view of an agent.
A beginning agent wants to show lots of sales. For a beginning agent, you want to list all those four figure sales. At this point in the agent's career, you want to show you're a player.
But after a few years, the agent starts getting some bigger sales, some five figure and maybe even a six figure. Let's say an agent does twenty deals, 1 six-figure, 2 five-figure, and the rest four-figure (17). If you were an agent, what would you put in PM? Would you list all those four-figure sales? Or would you list your six and five figure, and maybe half of your four-figure.
And then a few years later, what would you want to list? If you're now known as a top agent, how much would you want to list a four-figure deal?
Further, some deals are multi-book deals, and you only announce the first one.
Publishing Marketplace is a good resource, but any self-reported sales figure has to always be taken with at least some salt. Some with a lot of salt. Listing sales figures for an agent is a marketing tool. It shows writers who is who, but it also shows editors. My guess is only a very few agents list all of their sales, and probably only for a short period in their career.
Also be aware that some writers probably don't want to be listed, and some editors as well.
Best of luck,
Jim Clark-Dawe