OPM Standard cost at this level has material cost?

I created a simple Formula and recipe

Product has 2 ingredients

PJ Ingre1 Cost 100 ( It has pure material cost only)

PJ Ingre2 Cost 200 ( It has pure material cost only)

My recipe has a routing with a single resource

Expectation:

After the rollup. Lower level cost should be material cost ( Yes it does , so good here)

This level cost should be resource cost only is my expectation

However , I see this level cost includes a random material cost of '1'. Wonder where would this come from?

Regards,