x.x.... Brain electricity just died out from reading that... Uh... details?
OK, I'll explain. When you have residential service, 99% of the time unless your ISP is special, you will have a Dynamic IP address which can change at any time, whether the modem loses the connection, either sync wise or auithentication wise, or if the network just decides to change it. Now, in the case of my ISP Verizon, they have IP blocks assigned per area from what I've seen. Now, your ISP may be a different case, but if the IP address assigned to the No-IP account's name is not your IP address but used to be your WAN IP address, it was probably assigned to someone else wherever and had to go over your ISP's network to get to that connection, which adds latency. When you updated the No-IP record for your IP, if your IP had changed from the old assigned IP, the new IP now points to your connection. Since your modem/router is holding your WAN IP address, it will now route to your modem/router instead of going through the network and out, as your modem/router uses NAT, and it sees that it owns the IP in which the data has to go to.
So simply said, if your IP changes, the request for data will go to wherever your old IP was assigned to. When the account syncs, your network will see that it owns that address and will try to route it someplace inside your network. In this case, since your network once the No-IP account was updated saw that it owned the IP, the ping you sent was received by your router, and your router replied back.
And on the updater park, since No-IP has an updater program that runs on startup (preference), it will probably try to sync up your new IP address with your account. Pretty much, you can set that to do it either manually, or you can keep the program from opening when the PC starts.