The US has several distinct markets.  What you say holds true for the Gulf Coast and Midwest markets.  However, the Atlantic (Northeast), California, and Hawaii are all Brent markets.  It doesn't matter what WTI trades for in any of those markets.  And there is always variability in deliveries with wild fluctuations of week to week amounts received in these markets.  It just depends on when the ships arrive, and some have quite long voyages.