It looks like there is a very high volume of mentions of that account in that 2 day period, so it is likely that the API is capping the data for performance reasons - and also possible that a lot of them are lower-quality or blocked or muted accounts that are filtered out from the mentions before being presented in the API. I’ve verified that mentions of my own account seem to reflect an accurate / expected number.
For full fidelity results you’d want an enterprise API like PowerTrack (for realtime mentions) or 30-day search. Using the latter I just found over 2 million mentions of that account in the timeframe you’ve referenced.
