Can anyone tell why they're having instantaneous launch windows? AFAIR when NASA was flying Shuttles, the window had a significant size.
The ISS is moving at 7.7 km every second; if you miss by 10 seconds, you would need to (roughly) drop your orbit by 10km, wait 3.6 hours, then climb back up that 10km to make up for it.
Matching position is even harder. The ISS isn't equatorial. It swings north and south of the equator in a sinusoid.
It's probably possible to have a computer or pilot dynamically respond to all these things to make up for them. But easier to just deal with it by waiting a few days.