Correct. And the separation distances you were estimating were if the two players were running side by side without any contact.
Add physicality to get a WR off their route, superior DB technique/skill advantage (i.e. better ratings), alignment (e.g. starting with a 5-8 yd head start off LOS), and you can quickly understand how even a 5 SPD difference--which some Madden bros will want to be an automatic 4-5 yds of separation and consider that to be "much more realistic" (watch for this phrasing when guys discuss low threshold)--can on some snaps mean NO separation.