The Zorin OS 18 r2 revised isos were released shortly after the r1 isos, primarily to include a more robust fix for the E:Sub-process /usr/bin/dpkg returned an error code (1) error while installing Zorin OS 18 when the "modern NVIDIA drivers" option was enabled and newer Linux kernel versions are available.
When we release revised sets of isos we usually include a suffix (like -r2) to the file names of the free editions of Zorin OS. We do this in case there are any issues with deleting/overwriting the old iso files on some of the free mirror servers, which are run by third-parties like universities and organisations around the world.
As they can't store multiple files with the same name in the same directory, we need to differentiate them on the mirror servers by changing the names of the new isos, which we do by appending the "-rX" suffix.
However, as the Pro edition iso files are hosted on servers administrated by us, we can safely overwrite the older iso files with the newer revised isos there. As a result, we typically don't change the file name for the Pro edition iso files in order to maintain continuity and to avoid hosting multiple old & redundant files.