Indeed the weight is very different for the different components, especially oils, even castor oil will vary quite a bit depending on what additives they might have used.
Methanol and nitro can change as well depending on supplier. That's why you need the MSDS for the product you're buying so you can use the density that they have measured accurately. The whole point in using weight as a way to measure is that you don't need to compensate for temperature.
I believe there have been many posts and threads on this subject here, so some of the misconceptions found in this thread, as well as other useful information is out there.
