Because every fresh pepper varies in heat, every batch of sauce is different in heat level, even if the recipe isn't changed.  When a company gets a Scoville test done for one batch of sauce, then claims that heat level for years to come, the only thing that's guaranteed is that it is NOT accurate.  It's either milder or hotter, but statistically improbable to be that exact heat level again.  Tests run on Tabasco sauce can range from 2,000 to 5,000 SHU's per bottle, that's a variance of up to 250% in the single classic sauce, and it's totally normal depending on crop conditions. We use our own heat rating system as a loose guide to let you know what kind of heat you're getting into.  A 4 on our heat scale is relatively close to a Frank's Red Hot for heat.