This one is probably related to the well known property of human short term memory which states that we can remember about 7 plus or minus one things. This is the reason that phone numbers are seven digits for example.

Err, no and no. That was a hypothesis from the 50's that has since been invalidated, and many (most?) places in the world do not have 7-digit phone numbers. Memory is "chunky". For instance, you can easily remember phone numbers in your same area code even though they probably have 10 or 11 digits.