Multiple Invoices even when there is single fulfilement set

Hi,

I have done the setup for Order Type, so that one fulfilment set is created for all lines. It is working just fine.

But, I have seen cases, where there are multiple invoices even though there is single fulfilment set. If anyone has face this issue before please let me know possible reason. Please note, it happen only few times. Most of the time it work just fine.

One reason that I thought of, was , Invoice grouping rule, but it is setup as based on Sales Order level.

Not able to find proper reason.

Thanks
Ashish