Uniqueness is based on an UUID which is bounded to an OS user on a computer. The UUID is shared between all IntelliJ-based IDEs on this computer. The UUID doesn't change during IDEs updates. However, the UUID will be different if an IDE with one license is installed on various computers.
If the "All Versions" mode is selected (as shown in the figure below) then the plugin version is ignored during statistic calculation. E.g., the downloads of version 1.1 and version 1.2 with the UUID=123 are non-unique.
If there are several downloads with one UUID in different timeframes then in each timeframe there will be one unique download. E.g., the daily statistic has several downloads with UUID=123 in July and August. This will be counted as 1 unique download in July and 1 in August.