Revaluation Surplus – Topsy Co

The following trial balance extract relates to Topsy Co as at 30 April 20X6:

Land at cost $800,000
Building:
Valuation at 1 May 20X2 $1,500,000
Accumulated depreciation at 30 April 20X5 $90,000
Revaluation surplus at 30 April 20X5 $705,000

On 1 May 20X2, when the carrying amount of the building was $750,000 it was revalued for the first time to $1.5m and its remaining useful life at that date was estimated to be 50 years.

Topsy Co has correctly accounted for this revaluation in the above trial balance. However,

Topsy Co has not yet charged depreciation for the year ended 30 April 20X6 or transferred the excess depreciation from the revaluation surplus to retained earnings at 30 April 20X6.

In February 20X6, the land, but not the building, was independently valued at $950,000. This adjustment has yet to be made for the year ended 30 April 20X6.

What is the balance on the revaluation surplus of Topsy Co as at 30 April 20X6 after the required adjustments have been made?

Solution was given below

The correct answer was $840,000

At 30 April 20X5 $705,000
Increase in value of land in the year (900,000-750,000) $150,000
$855,000
Annual transfer to retained earnings
Depreciation on revalued amt ($1.5m/50)

Depreciation based on historic cost
$(15,000)
At 30 April 20X6 $840,000

My question is how does the increase in value of land ($900,00) deprived?