Some NAS (Network Attached Storage) devices will wrongly change the last modification date & time of a file whenever the file is opened, even if it is just for reading the contents (without any change to the contents). This can mean that any attempt by SyncBack to use the 'slower but more reliable method of file change detection' (also known as 'hashing', which involves reading the file's contents to calculate a signature hash value to use in comparisons) can unwittingly trigger the NAS into wrongly updating the last modification date & time as it does so. If this is the case, and updating the NAS firmware (always recommended) does not solve it, then you must switch off the slower method of file change detection.
Note, however, that many NAS devices are also unable to keep the correct last modification date & time of a file when a file is copied to the device anyway. It is instead usually set to the current date & time. If this is the case then the only option is to use the Fast Backup option with SyncBackSE/Pro (and never force a rescan). It is also highly recommended that you check to see if a new version of the firmware is available for your NAS device.