Perhaps the DTKs can sustain the higher speeds for longer, so Apple limited the top speed for heat reasons. Just because an iPad Pro can hit 2.5GHz doesn’t mean it sustains those speeds for very long.I can understand not using the "efficiency" versus the "performance" cores, but why would Apple underclock them in a Mini compared to an iPad Pro? There's more room in the Mini enclosure for air flow so there ought not be a thermal reason and it's always plugged in to a wall socket so saving power ought not be an issue either.
In any case, Apple did say the DTKs have some limitations that won’t be present in the consumer products. Functionality and stability are more important than raw performance in a developer kit.