I wondered the same thing till I went to Reno for a visit. Believe it or not what he has is a book with all of his carburetor recipes written down, various combinations of bleeds, jetting and metering block calibration all developed after many years of dyno and track testing etc. Sometimes what runs good on the dyno won't race or drive very effectively. Some of this stuff stays about the same and other stuff changes quite a bit over time. Bobby has a huge list with every carburetor listed by serial number, what calibration was used, what booster and what application.
A really good carburetor package will run very good on a variety of engine combinations and the better the package the better it will do this and the less engine specific it will be because the fuel curve is more controlled from conditions of very low, moderate or very high flow for a given design and it will only require some minor tweaking to be spot on with any given combination. He'll adjust the carburetor package a little one way or another, setting it up somewhat differently for drag, circle track or street to get it really really close out of the box for the intended application it is quite amazing.
Usually carburetor builders who play like their stuff is very engine specific and won't run well on something else, a Ford big block instead of a Chevy for instance really just don't have a very good package for a specific carburetor.
What the engine test does is verify that all the circuits are working, the float levels are correct, accelerator pumps work, no leaks etc.
While I was there we did two gas 4412 packages, one tech legal and another with a modified booster as per the rules. The difference on the test engine was quite astounding with the tech legal 4412 sounding good and crisp but the modified booster version sounding just that much better response wise.
Nothing about the carburetor build process is very high tech but it's very time consuming with an incredible amount of hand work. The metering package involves lots of careful compromises and precise work. Placing and sizing the restrictions in the emulsion well is both art and science when it comes to getting something that will run well at many throttle positions, rpm, engine loads etc.
Any sufficiently advanced technology is indistinguishable from magic --Arthur C. Clarke