Why is the answer for 2022 income recognized - 24 months = $18,000?

I thought from 6 months of 2021 and 12 months in 2022 it would be a total of 18 months so 2022 income would only be 12,000 with the remaining $6,000 to be recognized in 2023, because this is accrual basis.

Can you please explain why I'm wrong and provide any tax knowledge thank you!

Bigham Corporation, an accrual basis calendar year taxpayer, sells its services under 12- and 24- month contracts. The corporation provides services to each customer every month. On July 1, 2021, Bigham sold the following customer contracts: Length of contract Total Proceeds 12 months $14,000 24 months $24,000 Determine the income to be recognized in taxable income in 2021 and 2022. Length of Contract 2021 Income 2022 Income 12 months 7,000 7,000 24 months $ 6,000 18,000