I'm afraid that is not an adequate explanation. Obviously, assets coming off decrease depreciation, but if a company is rapidly buying new assets, then those should be getting included in depreciation. So the net result when a company is growing PP&E is that depreciation goes up. The ratio of depreciation to PP&E should not be coming down. It's almost unheard of to see depreciation going down while PP&E is going up. See if you can find any other example of this happening.