For "open" read "accessable" or better: "unfiltered". Next to that nmap uses it's own number-to-port mapping similar to /etc/services and just like the services file it's a *static* mapping. So, to extract information and confirm, if a port is "open" and there is service bound to it, use the version scan option. BTW, scanning localhost uses loopback which usually is excluded from filtering in the firewall and so gives a skewed picture of what is accessable. Best way is to scan from a box that's not in your LAN or use on of the free online services.
"a" vs "l"...