TI ADS1248 24bit ADC app note error
They're using a ADS1247 for the ADC in that app-note.

This is a 24 bit ADC, so the maximum output count is \$2^{24}\$ or 16,777,216. Considering their values are ~8,380,411, which is very close to \$\frac{2^{24}}{2}\$ (which is 8,388,608, so the offset is 8,197).

Based on that fact, I would assume that they're simply doing a improper twos complement conversion, which would effectively do exactly what you describe in the question - shift a range of +/- 8M to 0-16M.

The ADC in question (ADS1247) does not seem capable of producing a non-signed output, so I have no idea how they're getting those values, but they wouldn't make sense unless they're converting their signed readings into unsigned numbers somehow.



Basically, I suspect your thinking is entirely correct. What they're showing looks exactly like what you would see if one was improperly converting a signed value to an unsigned value. 

Unfortunately, they don't detail the digital end of their test-setup in any way, so there is no way to know for sure.