In the movie, it is stated that Replicants were designed to live for four years. The stated rationale behind this is that the longer the Replicants live, the more human they become, and it would be increasingly difficult to detect Replicants (via an emotional response in the Voight-Kampff test).

Why then would Dr. Tyrell develop the Rachael prototype, which drastically increases the difficulty (20-30 vs. ~100 Voight-Kampff questions) in detecting Replicants?