In my experience Rainmeter is quite accurate and I indeed am talking about the same thing you are. However, let me explain again.

From the manual:

"Update
The update time for the meters. The value should be given in milliseconds. The default value is 1000 (i.e. one second)."

Increase that, or increase the updatedivider of your measure, if you want to keep your other measures updating as fast as before.

And string measures have this option:

"Scale
Scaling factor for the measured value. The measured value is divided with the scale value, so in order to get 1000 times smaller values just set the scale to 1000. If the scale value has a decimal point (e.g. 1000.0) the resulting measured value is displayed as floating point value with one decimal."

You use the scale in any case if you want to see kb while it reports bytes (in this case you divide with 1024, see?). If you halved the update frenquency, you could now divide by twice the value instead (2048?) and get a slower-updating but steadier scale.

Read the manual through and you understand what you have to do.