It works because the manifold isn't tuned. Normally in a tuned system you have the header consisting of the 4 primary pipes from each exhaust port, of a particular diameter and length, and coming together into a collector area, also of a particular diameter and length.
In theory, everything that controls the actual flow behavior and tuning of the exhaust is done via the header/collector only. Everything after this is essentially meaningless, if done right.
Now on most stock setups, there really is not much for tuning. Most primary piping is not equal in length, nor is there any real collector area nor any termination of the collector piping. The whole exhaust turns into part of the "system," and it can influence the overall behavior.