It's hard to guess since you're haven't mentioned an example. For instance, in some cases Disney releases the Blu-ray/DVD combo pack first and then releases the DVD only a couple months later. This is an attempt to encourage consumers to buy the blu-ray (and spend extra $ on the combo pack).
It's highly unlikely there's a release with the feature on DVD only packaged together with only the extras on blu-ray. One of those two blu-rays is probably a blu-ray of the main feature.