The SPM reporting is independent to the Overlays reporting -- though they are related, since Overlays is powered by SPM JS code and the JS code will fire to load the Overlay and when a user visits a page. The SPM report is specifically accumulating the metrics based on the onsite JS code is firing when users are browsing the site and also how many times a user clicks on content from an SPM section. The Overlay report will report back specifically when the Overlay was launched and if users converted.
You wouldn't necessarily use the SPM report to see how Overlays are performing. The Overlay report are the best metrics to determine how the effectiveness of the Overlay.
Note: to access metrics for SPM and Overlays, they must be first enabled for your account. Reach out to your Customer Success Manager if you are interested in SPM or if you qualify for it to be enabled.