It doesn't keep growing because the sum is multiplied by 99 then divided by 100. In other words, you take 99% of the 100 point sum and add the new value (which is 1% of the total). As I said, it appears to me to be a bit smoother and more laggy than a genuine 100 point moving average (which sometimes can be a good thing), but it is only 3 steps and two operands to create a (pseudo) moving average of any number of points - to the limits of a 32-bit integer.
However, I'm going to study your LargeFilterJT a bit.
By the way, that vlp file appears to have been saved with 9.8.64, but you seem to be using the version from before Unitronics fixed the database issue to prevent it from being opened in 9.8.31. You should remove that version and re-download & install.