It's the speaker!

Whether the speaker chooses the present stem or the aorist stem does not depend on a general rule, but only on the state of mind of the speaker. He visualized the situation and the action ordered in some way or other, and he makes his choice accordingly. Consequently, the dramatic content of an imperative can be determined only from the special context.—The Greek Imperative, page 43

<idle musing> Remember, we're talking about Classical Greek here, not Koine/Hellenistic. He'll get to those in a bit. He gives numerous examples to defend the position; I'm moderately convinced—tentatively. How's that for equivocating? : )
</idle musing>