IAS 16

The question is,

A machine was purchased on 1 April 2007 for $120,000. It was
estimated that the asset had a residual value of $20,000 and
a useful economic life of 10 years at this date. On 1 April 2009
(two years later) the residual value was reassessed as being only
$15,000 and the useful economic life remaining was considered to
be only five years.
How should the asset be accounted for in the years ending
31 March 2008/2009/2010?

and the answer for 31st March 2010 is,

31 March 2010
As the residual value and useful economic life estimates have
changed during the year ended 2010, the depreciation charge will
need to be recalculated. The carrying value will now be spread
according to the revised estimates.
Depreciation charge:
100,000 – 15,000 = $17,000 per annum
5 years
Income statement extract 2010 Depreciation $17,000
Statement of financial position extract 2010
Machine (100,000 – 17,000) $83,000

Why not deducting 15000 (residual value) from the carrying amount?