iExpense - Project allocation page is not enabled Project & task code text box

we are using an iExpense template, which has around 30 expense lines (each line is assigned with PA_EXPENDITUIRE_TYPE).

for ONE  peticular expense line, Project & task code text boxes are NOT enabled on Project allocation page (step 4 of 6). It looks fine for all other expense lines of the Template.

i checked all setups for the expense line and do not see any thing different for this perticular expense line.

any clue....what is the reason for not enabling Project & Task boxes for this expense line ?