Why a payment which is not cleared with any open item is split into different main transaction

Hi Experts,

I have a situation where a payment lot of 110,13- Euro  was split into 2 different sub-items with different mains (0060 and 0600) and was open on account. i.e. was not cleared with any other open item. What could be a possible reason of this split into different mains. Where in config can I look to see this split into different mains.

Any inputs are appreciated.

Thanks,

Priyanka