AS 22

theoritically we have to consider dep. only

If you consider the depr. for the year, you will arrive at current year provision for deferred tax. i.e., the extent of deferred tax to be created for that particular year. Whereas, if you consider the WDV, you can arrive at the closing balance of the 'Deferred tax' A/c. To be clear, WDV reflects the correct balance considering the errors, if any, in the previous years. It doesn't mean that, taking the depr for the year will reveal incorrect amounts. Consideration of depr for the year will depict the correct figures, only if the previous calculations are also done correcly. You may try some illustrations given in any book.