The only thing I can think of is this:
Say the orbit speed is 30 000 km/h (don't know - just did a rough calculation). Say you can't be more than 1500km out (3 minutes cruise time) as you won't have enough time/fuel to get to your docking point if you're out by more.
If the perfect time to leave is 17:30, you can leave between 17:27 and 17:33 and still be within 1500km. Any more and you're not going to make it.
Just my 2 cents worth.