CPF - OW wage type added to AW (/303) in retro (Process clarification)

Hi,

My scenario is as follows..

April:-

WT-9000 -Employee basic salary -5000 SGD u2013Ow

WT-9100 -Night shift allowance -600 SGD u2013OW

So total OW is u2013 5600 sgd

OW per month Capping at 4500 u2013

So employee u201315.5 % - 900

Employer u2013 20% - 698

May:-

WT-9000 -Employee basic salary -5000 SGD u2013Ow

WT-9100 -Night shift allowance -600 SGD u2013OW

WT-9100 -Night shift allowance (new record for April month u2013 thru retro) - 200 SGD u2013

System taking this as AW thru rules RC10.

OW Capping at 4500

So employee u201315.5 % - 900

Employer u2013 20% - 698

Even though the wage type is marked as OW during retro it is getting grouped under AW(/303). Its happening through standard PCR RC10.What is the business logic?

Additional information u2013 When I have checked the retro for basic salary also the difference amount is getting calculated as AW.

Regards

Thomas