I'm not surprised. For one thing, out of position penalties are not turned on as far as I know. Speed is also not a determining factor for coverage, especially short and very short.
After doing some testing with the new update, I think the best way to cover pass is with a 4-3. It seems the CBs get averaged and then they influence the whole field from sideline to sideline and from short to deep. If you play more lower rated CBs, I would bet that your coverage would suffer because of the lower coverage rating.. You can even run a 4-3 agains the shotgun successfully and since one CB will effectively cover two WRs for you.