You can search the forum for “unexpected difference” to see other discussions on this topic. But to quickly answer your question, I’d try deleting just that backup version.
Quick instructions:
Click on your backup set in the Web UI, click the Commandline link, then select “delete” from the Command dropdown. Empty the contents of the Commandline Arguments text box. Then scroll to the bottom and pick “version” from the Add Advanced Option dropdown. Enter the number 12 for the version and click the “Run delete command now” button.