The physics or geometry of the situation is that an 8" throat takes the same amount of water in volume to accumulate 1" of rainfall.
What goes on inside has zero to do with the amount needed.
However as you allude to, there are lots of adjustments inside and the gauge itself needs to be very level for the tipper to function.
One winter I spent a lot of time with the kitchen sink and a very slow dripper to make sure no drops were lost during the actual tip when the divider was vertical. Even with precisely measured (by weight, which any cook knows is more accurate than volume, especially of that amount around 800 cc or ml or grams, all within the range of one another for practical purposes) and I got, from the same gizmo and run the same afternoon, variation of a tip or so, which I thought was very close.
The rain rate makes a difference. The amount of gunk from the atmosphere washed into the funnel and making the water stick to the surface and not flow into the tipper is a concern (but not for volume calculations).
I am amazed that even using chemistry techniques of volumetric measurement, the amount in a beaker (assuming it is accurately calibrated and almost none are) is way off in percentage.
Using a Pyrex measuring cup? Forget it. Get a scale, do a bunch of known weight measurements to see how reproducible they are, then use that to figure out how many ml you need. Getting it all out of the holding container is another error.
The number of errors adds up in a hurry.
This is all for the lab calibration. Obviously being out of doors, even within a few feet of one another is also fraught with errors, as one can tell from watching sheets of rain fall during a heavy storm and seeing wind effects, so the fact that two gauges out side measure differing amounts also is a factor.
The pictures of the old National Weather Bureau rain measuring stations with their 12" throats and what I called the "Green Giant Loin Cloth" of the bigger circle of hanging metal sheets to try to reduced the effect of wind comes to mind.
To me, the biggest error is the mounting of the device out doors. Inside, of course, should be a highly controlled test, with it being precisely level, and testing the unit to make sure the exact number of drops or calibrated valume causes the tip to occur on each side. Sort of like a pendulum clock being out of vertical when the tick and tock are not the same (for most big clocks) can throw things off.
For the 8" volume needed to give precisely one inch of rain, take 8" diameter, get the area by (pi x radius (4") squared) = 3.14*16 = 50.26 square inches, then times 1" of rain which makes for 50.26 cubic inches, and then convert to cc which is about 1 ml which is about 1 gram (16.39 cc is one cubic inch) = 823.76 cc, Since one cc of water weighs 0.9982 grams, then it is pretty much within the range of 1 to 1.
Scales we all have are better than they were, but none of this would pass even basic chemistry in high school, say nothing of college and of course quantitative analysis labs, where we could measure the weight of a finger print. But I doubt we need that accuracy. Be sure to carefully measure, with an accurate ruler, the throat of your collecting funnel. It may not be exactly 8", although RainWise is pretty good. High Sierra and Campbell and Young and Texas Weather Electronics (the Gold Funnel ones) have never been off when I've checked with the best rulers I have. One old brass official (antique) NWS gauge has an almost knife edge to collect the rain, and assumes no wind.
Check the level. Check the equality of each bucket tip, check for debris.
But when you have a consistant difference from one type of gauge (Stratus) to RainWise (tipper) then you'd be correct in thinking something, somewhere is a wee bit off.
Oh, don't forget evaporation from collector gauges vs. tippers.
The list goes on and on.
I'd be interested in hearing what other's experience has been with trying to make sure their gauges are the most accurate they can be.
Dale
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