When you download a file, you find sources who have some of that file's data to give. Each source has a queue of clients (like you) who are waiting to download files from them. When you first connect to a source, you get on the end of the queue.
Each source uploads file data to a few of the clients that are at the front of their queue. When the source has uploaded to a particular client a full chunk of the file, or uploaded for a fixed time period, then it moves that client back to the end of the queue and starts uploading to the next client at the front of the queue.
So, if you happen to find sources with short queues, you will advance to the front of the queue quickly (maybe immediately) even without credits. Credits only allow you to advance up the queue faster.
So, as stefanero said, it's a matter of luck. Your friends must be downloading files for which there are lots of sources with short queues.