Why do we forget when neurons in our brain store every bit of information discovered by us? Also, on what basis is the IQ test of a person calculated?

Humans have basically three forms of memory: Sensory, Long-term and Short-term. Long-term memory is just like hard-drive space. Similarly, Short-term memory functions like a very small RAM. This Short-term memory is capable to hold only about five to nine (seven is an average) items at a time.
The Standard-Binet is probably the most commonly used IQ test in the United States. Th purpose of this test is to determine the participant's intelligence quotient (IQ), which is a ratio of the child's mental age compared to their chronological age multiplied by 100. A normal score is 100, with scores falling below 100 as being below normal, and scores above 100 as being above average.