Here's one reason why the company might legitimately be able to claim that they can access more data than you can find online with 192.com.
192.com's online data is based upon the edited electoral roll. Anyone who marks the relevant box on their electoral registration form will be excluded from the edited roll. However, somebody who has only marked the box for the first time this year will still be listed on last year's edited roll. (So, as long as they haven't moved, they can be traced by using the old database). 192.com allows you to access older edited rolls but the Information Commissioner has ruled that they must not show the details of anyone who has opted out of being included in the current roll.
However, the firm behind 192.com also sells their database (as 'UK Info-Disk') on CD. Anyone who has got the CD they published last year will still be able to see the data which the Information Commissioner has prohibited from being displayed online. So, as long as they've got last year's CD, the tracing agency, which you refer to, can legitimately claim that they've got more data than is available online.
Chris
PS: The data protection implications of using those older CDs are very much of a grey area. For example, Camden libraries won't even stock the current CDs, because of data protection concerns. However, Suffolk libraries staff were happy to spend 40 minutes looking for the relevant software disk, so that I could view an older version of the electoral roll.