Parameter Analysis

Good morning, I’m analyzing the InternetGatewayDevice.LANDevice.1.WLANConfiguration.1.TotalBytesReceived parameter over time, in 10-minute collections. In theory, the subtraction of a current collection by the previous one should be the total number of bytes received in 10 minutes, correct? But in this subtraction I am having negative values ​​at various times, what could be happening?