Das Boot is a movie - and tries to show a more or less accurate picture of the U-Boot warfare. Due to drama or limited information the movie Kaleun may use less optimal orders - just to entertain a 'non-expert' audience.
As long as a DD closed in at high speed, a Kaleun would have ordered high revolutions to gain distance, but would have dropped back to slow after some time to enable his own hydrophone operator to listen for the actual speed of the DD - simply to adapt his own tactics.
If the DD would have had slowed down the Kaleun would have assumed it tries to establish contact and would have rigged for silence and very slow speed.
If the DD would have had kept its speed - rushing the last known position to drop depth charges - the Kaleun would have further tried to gain distance.
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