You can look at their website for all of their advertising, facts, figures and assertions. They do not claim to pick up results earlier than any other test.
All other tests are very open about their capabilities and say exactly the same thing about early detection.
For the record:
"In clinical testing with early pregnancy samples Clearblue DIGITAL Pregnancy Test with Conception Indicator gave the following results: 55% of women received a ?Pregnant? result 4 days early, 86% of women 3 days early, 97% of women two days early and 98% of women 1 day early" so yes it can work early but so can many, many others.
If you google 'hpt sensitivity charts' there are many, many sites with a breakdown of the hcg detection levels of tests. Many have exactly the same accuracy as the clearblue including supermarket own brands, first response, ebay cheapys, a couple are even more sensitive.
Clearblue are the market leaders and that's why you see more of their adverts than anybody else's.
Their key claim over other brands is that they use words, not lines so are easier to interpret, but certainly not better, faster, earlier etc.