revaluation surplus

A business owns a building which it has been using as a head office. In order to reduce costs, on 30 June 20X9 it moved its head office functions to one of its production centres and is now letting out its head office. Company policy is to use the fair value model for investment property. The building had an original cost on 1 January 20X0 of $250,000 and was being depreciated over 50 years. At 31 December 20X9 its fair value was judged to be $350,000.
in this task above, it is calculated: carrying amount to 30 June 20X9 is 202,500. it is ok. and revaluation surplus is 147500 because fair value is 350000. it is ok too

but it is unclear to me, in this task solution it is said that the building will be subjected to a fair value exercise at each year end and these gains or losses will go to profit or loss. If at the end of the following year the fair value of the building is found to be $380,000, $30,000 will be credited to profit or loss.

1) WHY 30000 IS RECOGNISED IN PROFIT OR LOSS, IS NOT IT CREDITED TO REVALUATION SURPLUS? because if we revalue assets and we have positive excess we credit that difference to revaluation surplus. is it? but why in this task is it said the difference credited to profit or loss, not revaluation surplus?

thanks in advance