In software development it’s quite common to think “I finished” and then discover that you’re far from finished. You still have to polish the result.
And the funny thing is that finishing the last part of the work, that 5 or 10%, is much more difficult requires more focus and attention because you need to polish things up, to be precise, to look out for small errors only the best users would find.
It requires people to think outside the box, to imagine where they could have put an error, what could go wrong, to make it pretty and respect the work and effort the designers put into the prototype so that it can become pixel perfect.
Going from 90% to 95% is an astonishing work, going to 99% is even more beautiful and challenging and it’s what makes software development so difficult.
You can settle much more early than 90% and feel like you’ve finished. The hard part is keeping up with the schedule and deliver at least a 95%.