I always thought of it like this:
Commercial H2SO4 is produced by burning elemental sulfur, which is then run through scrubbers and catalysts, and added to water to get 100% H2SO4. But given that H2SO4 is just about one of the most hygroscopic compounds around, it would take an absolutely anhydrous environment throughout production and packaging, as well as shipping and the actual use of it for it to stay 100%. This is, for all intents and purposes, impossible. So the chem companies cover their asses, and put 98% min. on the bottle. It might very well be 100%, but there's no guarentee. And I highly doubt it. So, in order to get 100%, they add SO3 to it, and they have it, but they also have some dissolved SO3. So I would guess that you could have some chem company do a custom synth, for an astronomical amount of money, and get you some 100% H2SO4, or you could just add some SO3 to some 98% H2SO4, and just use the best analytical scale you can get, and use an absolutely anhydrous environment.
Who is that masked man?