Minimum flight separation does not correctly allow flights to be scheduled at the set value

I have set the minimum flight separation to 15min. But i cannot put the flights closer together than 20min

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COLLECTED HARDWARE DATA
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Device
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Model: System Product Name (System manufacturer)

Operating system: Windows 10 (10.0.0) 64bit

RAM size: 32 GB

Battery status: Charging
Battery level: -1

Processor Device
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Type: Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz
Core count: 8
Frequency: 4 GHz

Grahpics Device
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Vendor: NVIDIA
Type: NVIDIA GeForce GTX 1080 Ti
Memory Size: 11 GB

Screen Summary
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Full Screen: True
Resolution: 3440 x 1440 @ 100Hz