Actual sales data.
Lexus typically has much better than avg. resale value. I wouldn't read too much into one car that you pick out of auto trader. Moreover, I wouldn't put too much into the resale values of the IS vs the rest of the Lexus vehicles. The 2001 IS got clobbered with respect to resale due to a number of things including: significant improvments to the 2002 (only one year after being introduced), the 2001 was the initial year for the car, immediate stronger competition from Audi, BMW and now Infinity, subsidized finance rates from all manufactures including Lexus, and price reductions on some packages for the 2002.
Lexus vehicles typically hold their values very well.