When you click on Known Spam you will see a a graph of message counts per day instead of a list of messages. The caption at the top of that screen says,
"All Known spam that is sent to your address is currently being deleted. The graph below shows how much spam has been caught by Known spam Blocking over time."That is the default setting, to count it and delete it. Our false positive rate (flagging a good message as spam) is less than one in one million, so this method is best for most users and allows us to provide better service without storing all of the 100+ million spam messages we process every single day.
You can change that setting by clicking on spamBlocker / Settings and scrolling down until you see the setting that says, "Delete Known Spam Immediately" and just change that to Off. Then you will see a message listing similar to other folders when you click on Known Spam. The most recent 100 Known Spam messages up to 10 days old will be saved in that folder. When a message in that folder reaches 10 days old it will be deleted automatically unless you move it to another folder. If the message count goes over 100 in less than 10 days, only the last 100 are saved.
One reason users get confused about the Known Spam folder is because of a bug in the message count shown beside the Known Spam folder name on the main Web Mail page. That is being fixed in the upcoming release. It should always show zero if the messages have been deleted, and should always show the correct count if they are being saved.