PhD Chemist here...
Some of the points made during this thread are spot on. The MSDS for the Interlux stuff must include the actual formulation amounts, but generally it gives a range to allow formulation variances and allows the vendor to protect the actual formulation. We do it all the time. 75-100% is a huge range for a specific component, especially one that has a wide range of boiling point, evaporation rate, etc. properties.
As far as blending your own, you can do that, but bear in mind that Kerosene is a pretty generic petroleum product and varies in composition and purity significantly. Same thing with Naptha, Mineral Spirits, Xylenes, etc. When burning these to produce heat and energy or used as a cleaning solvent, the composition isn't that critical but if using as a paint thinner, it could be. Generally, high purity, filtered grades (more expensive) are used when used as solvents in coating formulations. Your store bought stuff is probably one of the lowest grade (less purified, non-filtered) versions, hence the cheaper price.
While I am all for the "Less Expensive Approach" in a lot of cases, for thinning expensive formulated paint, I'd buy the specific thinner that is designed to be 100% compatible with the paint.