why two transfer order documents are created during SU creation?

hi,

The following statement by standard SAP said:

If a mixed storage unit is put away, a separate transfer order item is created for each material item in the storage unit.

However, I create a material document with two materials, and one TR is created for the two materials upon GR. Next, create a storage unit for these two TR line items, and system is able to group these two material into one storage unit, however, two transfer order document is created when I check the storage bin.

Question: May I know why two transfer order documents are created? shouldn't one transfer order document is created with two line items?

Please note one material having a palletisation maintain, while the other does not maintain palletisation in the material master.

Thanks

Tuff