Depreciation added to the accumulated depreciation is lesser than the depreciation on the system

Good day all,I noticed that Depreciation added to the accumulated depreciation is lesser than the depreciation on the system. For Example , the Accumulated depreciation on the system for the month of december is 6500, and the depreciation for the month of january is for example 500. I noticed that the accumulated depreciation calculated at the end of january is 6800 i.e (6500+300). Thereby not taking the full amount i.e 500. Thank you