Revenue

Dear Sir,
Could you please help me with this question?

“Scrubber provides contract cleaning services in commercial office premises. Scrubber
charges each business an annual fee of $1,200, based upon providing an agreed level of
service each month. In one office block there are twelve businesses which use Scrubber to provide cleaning services. At 1 April 20X5 four businesses had paid one month in advance and two customers were in one month in arrears. With effect from 31 August 20X5, one customer terminated their agreement with Scrubber, whilst two additional contracts were signed to take effect from 1 December 20X5. At 31 March 20X6, the same four businesses had paid one month in advance and two customers were in arrears by one month. Each annual service contract is regarded as a contract which gives rise to obligations which are satisfied over a period of time.
What sales revenue can Scrubber recognise in the financial statements for the year ended 31 March 20X6? ”

The correct answer is 14500$, here’s the answer:
Customers for a full year ((12 – 1) × $1,200) 13,200
Terminated contract to 31 August (5/12 × $1,200) 500
New contracts from 1 December (2 × 4/12 × $1,200) 800
–––––––
Revenue for the year ended 31 March 20X6 14,500

But what about the money paid in advance by four businesses and money paid in arrears by two customers? Why isn’t it included in calculating the revenue?