Downloaded core 0.15, I'm able to send and receive without issue.

But when I check the status of my computer at https://bitnodes.21.co/, I get an error message saying it is unreachable.

If I'm able to send and receive btc using the QT, shouldn't this website indicate that my port 8333 is accessible? Is it possible to send/receive via QT but my port 8333 is not accessible?