IELTS Writing Task 1: hyphen and no 's' before noun

 Look at the following (correct) phrases:

• over the 10-year period
• the charts show three 10-year periods

Now compare those phrases with two more:

• over the period of 10 years
• the charts show three periods of 10 years

Can you see the difference? Can you explain the grammar rule here, and can you think of any other examples?