Why are 20inch car rims bigger than 20 inch bicycle rims?

Are bicycle rims and car rims measured differently. The other day I had my 20 bmx bike next to my 16 inch truck rims and noticed that they were the same size. Even further I rode my bike across the street and compared them with my neighbors 20 inch he has on his car. The car rims were larger in diameter than my 20 inch bike rims. What is going on???
 
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