Nonstop Backup backs up whole folders repeatedly when nothing changes

I noticed that my backup disk was nearly full and found that the Tim Machine folder contains backups of files and folders that have not changed. For example I have a downloads folder that contains many large files, this folder is getting backed up a couple times a day and I cant figure out why.
I am also seeing "error occurred while opening this file" and "unspecified error" messages in the log file. What is causing these?

Thanks