Hi STP,
There are already some valuable replies, but I would like to add some details here. I always show the formulas like below to directly show that the base for the depreciation is different
Straight line service life: (Acquisition value - Scrap value) / Total periods during service life
Straight line service remaining: (Net book value - Scrap value) / Remaining number of depreciation periods
Initially, the outcome will be the same in case there are no changes in value or remaining service life. In case you are e.g. halfway through the service life, there can be a change to the asset having an impact on the value or remaining service life. After such a change, the depreciation amounts are different for each of the methods.
In case the initial acquisition value was 40000 with a scrap value of 4000, the depreciation base is 36000. In this example, we can also use a lifetime of 3 years with 36 periods. In the first 18 months, the depreciation amount for both methods was 1000 per month. In case you post an additional acquisition value of e.g. 7200, then the following 18 months, the calculation will be:
Straight line service life: (47200 - 4000) / 36 = 1200
Straight line service remaining: (21200 ) / 18 = 1177,78
Also, in case you change the remaining number of periods, the outcome of both formulas is different.