The most common source of this discrepancy is that we have no data for the users that have @mentioned you. Since we do not need this data to display a count in the graph, but do need it to show in the .csv (show the username) this number could be lower in the .csv. Another way to put it would be to say we have the @mention data but not the user data.
The mentions initially include non-native retweets (e.g. "Cool! RT @square etc..."), but these are filtered at a later stage.
Regarding the 0 count on the 20th and the 21st. The script processing the @mentions ran into a fatal memory error which was resolved yesterday, but unfortunately two days are now missing for some accounts.