The conveyor meter (and probably other "meter" valves as well) continuously resets (at an unknown and apparently variable rate) while receiving a green signal, causing it to send more materials than it's supposed to. Even very short pulses (less than 1 second) can cause it to reset while materials are flowing through it, doubling the amount of materials sent.
1. Create a conveyor system with a large amount of materials on a rail behind a conveyor meter.
2. Set the meter to a small amount of materials (ex., 5).
3. Connect a timer sensor to the meter's input.
4. Set the timer sensor to (for example) 10 seconds green, 200 seconds red.
5. Instead of resetting once per green input (and sending 5 items at a time), it continually resets (at a random / unknown rate) while the signal is green, sending a larger, variable amount of materials.
6. Changing the duration of the green input causes a different number of resets, but even very short durations can cause more than the selected amount to pass through the meter.
Resets should be caused by the transition from red to green, and should only happen once per green input, regardless of the duration of that input.
Alternatively (more in keeping with how reset lines work on chips, but probably less intuitive for players), a green input could cause the meter to stop working for its duration, reset the counter, and wait for a red input to start working again (this has the same practical effect, but shipping would resume at the transition from green to red, instead of red to green, with a pause during the green signal).