Cannot Insert new row in between the lines in AP Purchase distribution entry window

Hi,

Not sure why I cannot Insert new row in between the lines in AP Purchase distribution entry window:

AP - > Receiving Transaction Entry - > AP Purchase Distribution Entry window

From the menu I select Edit  all lines are enabled except the Insert Row command is greyed out!

This is not the case in the sales transaction side.

Is there any logic behind that restriction in AP?

Thanks in advance?