Time slot issue

here’s my current setup:

Session average duration 90mins
Shift: 17.30 until 22.00
Offset between reservations: disabled

Expected time slot on the booking form: 17.30 – 19.00 – 20.30
Actual time slot showing: 18.00 – 19.30 – 21.00 (wrong)

why is that?

But if change the settings to:
Shift: 16.30 until 22.00

Actual time slot showing: 16.30 – 18.00 – 19.30 – 21.00 (correct calculation)

The page I need help with: [log in to see the link]