I don;t think it really matters to the point we need to get so accurate as to strip them to nothing for weight. Yes you have variables. It does not matter what engine, tires, wheels, paint, etc...
If a racer builds his buggy to race but uses different stuff from 10 other guys the weights will be within 3 ounces.
It's just cool to know a "built" approx weight.
So if I build like I mentioned above and came in at 7 pounds 5 ounce and another racer built the same stock buggy but used a lipo, different tires, mag head speed, etc...he may be 2 oz lighter...maybe.