Why would anyone assume that the fuel mixing is done in the jug?? Whether by weight or by volume, this would be exceedingly difficult to get a reproducable mixture on a mass production basis. I would assume that batches of fuel are measured into a large mixing tank. After thorough blending, a bottling run would be done. Jugs may be filled by weight, by volume or by optical sensor. In the latter case, there could be some minor volume errors, but the composition would not be an issue.

There are lots of devices used to accurately measure liquid volumes - easier than by weight although possibly not quite as precise. Certainly it is possible to measure volumes accurately enough to produce mixes that are correct to within a fraction of a percent for individual components. I would also assume that if fuel component density varies significantly as a function of temperature, they would simply compensate for the differences on the basis of known (or measured) coeffecient of expansion. If a manufacturer did not have this basic level of control, I would suggest that they would produce a product that is inconsistant with respect to quality and concentration.

Bottom line - if it works for you, don't worry whether it is blended by weight or by volume; it can be done very well either way. If you get a "bad" jug of fuel due to reasons other than inappropriate storage (by you or your LHS), then steer away from that manufacturer!

Ross