The port scanner shows some open ports even though I'm sure they are closed
This typically happens when there's some proxy/firewall between you and the scanned device. For instance, some cell network providers proxy all network requests so all ports appear open. What happens in this case is that when the port scanner tries to connect to the scanned device, the connection attempt is captured by the proxy so in fact, you are connecting to the proxy and not to the remote device (the proxy then connects to the scanned device on your behalf and forwards the data to you).
Also make sure you know from where you are performing the port scan. For instance, if you do a port scan of your router to be sure no ports are open from the Internet, you should get the external IP, disconnect from your wifi and perform the port scan of the external IP. If you do this while connected to your wifi, you will find open ports of e.g. the web interface (80, 443) and possibly other services which are however only visible from your LAN and not from the Internet.