Unfortunately, there's no way to backtrack now. And we won't know what the real reason was either.
The only possibility I could think of is that you might have been using Versioning on Right? And you manually copied back an older copy (Version) of the PST file, which resulted in the PST file not being the most 'up-to-date'. Or, maybe you copied an old PST file from a wrong path altogether.
By the way, if you do not expect independent changes on Right (since you set the option 'Left always overwrites Right'), why did you initially set up SmartSync profile? Wouldn't it be better if you set it up as Backup or Mirror profile instead?
The only possibility I could think of is that you might have been using Versioning on Right? And you manually copied back an older copy (Version) of the PST file, which resulted in the PST file not being the most 'up-to-date'. Or, maybe you copied an old PST file from a wrong path altogether.
By the way, if you do not expect independent changes on Right (since you set the option 'Left always overwrites Right'), why did you initially set up SmartSync profile? Wouldn't it be better if you set it up as Backup or Mirror profile instead?