PowerPoint is performing a Fast Save. This save procedure is faster and safer than a normal save. A Fast Save appends only changes in your presentation to the end of a PowerPoint file instead of overwriting all existing content in the file. This information includes any changes made to the file and a record that the file was accessed.
The fast save information will be discarded once it has reached a certain size. If you would like to force PowerPoint to discard the Fast Save information, disable Fast Saves and save your presentation again. To do this, follow these steps:
1. On the Tools menu, click Options.
2. On the Save tab, click to clear the Allow Fast Saves check box.
3. Click OK, and save your presentation.