the changes in inventory level cause differences in profit as per marginal and absorption costing cause of overhead absorbed into the units.
in question only one o/h absorption rate is given. So are we assuming that the opening inventory that have been produced last year have absorbed the o/h at the same rate as that in the current year cause its very much possible to have different o/h absorption rate in the previous year by which the units have absorbed.
It is possible in practice that there is a change in the absorption rate, but in the exam always the absorption rate will stay the same. (But you should anyway be able to realise yourself what the effect would be 🙂 )