The variety of TaTs is driven more by the analytical requests...

The variety of TaTs is driven more by the analytical requests and test methodologies, they are not simply different price points (though the majority of analytical requests can be accomodated by a standard 5 business day TaT, with cost loadings for faster. 48hrs is often the fastest for certain requests). One of the tests I regularly request takes 5 days minimum as that is how long the accredited method takes. It cannot be done faster.

Known TaTs only really apply to common requests and using well utilised methods. Where the analytical request requires any ingenuity in test methodology, or the sample matrix presents unexpected complexities, the TaT provided is nothing more than an indicative time estimate, within which they hope to be able to provide a result that meets all QA and accreditation requirements.