There might be components that don't meet spec during testing, so rather than throwing away perfectly good assemblies, they can be derated, and the resulting product will be just as reliable and accurate as a full model, less the bandwidth and sample rate; but can be sold at a modest discount.
Most of the time, it's probably just for marketing reasons, and the assemblies are identical.
Assemblies can also be made intentionally with sub-spec parts (like a 300MS/s instead of a 600MS/s ADC), to save bill-of-materials cost directly.
Such models nonetheless might be overclocked to the regular model (because the manufacturers, in turn, do much the same thing with their products), but in that case you run more risk of problems, because there may well be good reasons why the manufacturer selected their parts for that speed class, and all that. The lower spec parts may generate crappy numbers (or outright gibberish) at high speed, or are okay at room temperature but fail at high temperatures, or dissipate too much power; and the lower clock speed suggests the opportunity to save a few cents on the heatsink in the reduced version, potentially exacerbating the heat problem.
Not all of these possibilities may be in play, but they're all options to the product engineer, and things to be aware of if you're looking to push them.
Tim