capital allowance calculation for AIA

Dear Sir.

Jogger question number 240 in BPP revision kits which is amended from exam question ( 12/08). i will copy below what exactly is written in respect to the capital allowance.

July 20. 2019: sold a special rate pool motor car (117,00)
July 31. 2019: Purchase motor car Co2- 105 11,800
Aug 15. 2019: Purchase motor car Co2- 45 9,000
Sep 30. 2019: Purchase Machinery 500,000
Dec 15. 2019: Purchase computer system 12,500
Mar 14. 2020: sold a lorry (8,600)

The motor car sold on July 20. originally cost more than this amount. The computer system purchased on Dec 15 has a precipitated working life of 6-years and relevant election has been made. the lorry sold for 8,600 originally cost 16,600.

The plant and machinery has opening balances on Apr 1, 2019 as below:
main pool: 26,600
special rate pool: 21,167.
the company will claim the maximum available capital allowances.

the Answer:
the answer of the above question was understood for me, except one thing, which i need your kind help on it, please:

for the machinery purchased on Sep 30. 19 they computed only 200,000 as AIA ( 100% capital allowance). The remaining 300,000 were added to the main pool and 18% was used to compute the capital allowance.
there is no indication that 500,000 is exceeding the AIA limit of 1,000,000 or even if its short short or long assets. The tax payer here is a corporate company and they have full year accounting period.

I really appreciate if you have any idea why this treatment of AIA ( 200,000 qualified for full capital allowance claim and only 18% for the remaining 300,000) as there is nothing mentioned in the exam report or the answer more than these figures.

Thank you in advance.

Omar..