OpenVAS has a quality of detection (QoD) setting, which is used to filter out some of the less reliable tests. Each vulnerability test in OpenVAS is assigned a QoD score based on its reliability. Scanning with a QoD of 70% is a common default in the cybersecurity industry. To see all possible vulnerabilities you can lower the QoD setting all the way to 0% and scan again. At 0% you would expect to see more findings, but with more false positives.
You can change the QoD for your OpenVAS scan in the account settings page.