This is probably just how Rainmeter calculates the traffic. If I understood correctly, it can only count network frames or TCP packets, so that if you are downloading something, the value may be quantized by as big numbers as 1500 bytes or so.

If you wish it to be less jumpy, you could always increase the update interval and divide by a larger number though (by default, Rainmeter updates every second and divides by an amount you specify, making the values fit the frame you wish).