We usually publish the Pollution Inventory data in the following calendar year e.g. 2018 data was published in 2019.
We recently changed where we store our datasets and a migration happened from the old repository to the Data Sharing Platform in the last few months.
September is likely the date when all the Pollution Inventory datasets were moved and this is what is recorded in the system.
If we have updated any dataset it will have a version number e.g. 2018 Pollution Inventory v2 and some text in the metadata extract.