Question re differences between data obtained via HAN vs DCC

Hi, This may be a newbie question and maybe even covered elsewhere, but I've been trying to understand this for a little while and it's still not quite clear to me.

Background: My supplier has told me that historic data (a period of about 3 months, around 9 months ago) is "missing" (they say they "cannot find it"). As a result I am having billing discrepancies. I don't really know what "missing" means, since my understanding is that the smart meter (SMETS2) stores data for 13 months anyway, so it should be there.

My guess is the DCC doesn't have it (for some reason) but the smart meter does.

I've just ordered the Glow IHD CAD to see if I can understand the data or problem better.

Here's the question:

Is the historical data that I can access directly over the HAN going to be the same as the data that the DCC has - in other words, identical for all the periods where the DCC wasn't missing my data - aligned to the same 30 minute periods, same timestamps, and not of a lesser quality etc - and could in theory serve as a backup and legitimately be used by my provider to bill me correctly?

Or am I misunderstanding some differences between data sent to DCC and data available to the Glow CAD/API ? and/or misunderstanding how historical consumption data is stored on the meter.

The context here is specifically the 30-minute consumption data, as used by TOU tariffs. (my provider is Octopus. They have been only somewhat helpful in the investigation so I'm trying to see if I can take things into my own hands before the 13 months is up and the data is lost forever)