Startups invest a lot of energy in getting into the door of well-known VCs (step 0 of the process). Strangely enough, some of them drop the ball later on wasting that earlier effort.
After you have convinced a VC partner, she has to take the idea to her other partners, often in some internal meeting without the startup being present. When a startup presents to a VC, mistakes are often forgiven, and gaps in data can always be filled with a follow up email. Usually the VC partner wants you to succeed.
Internal VC presentations are a bit more brutal. There is less time, people do not need to be polite to guests (people say what they think), and everyone expects the perfect pitch with all information available, there is no second chance. Missing facts often lead to a turn down.
Talking to some of my VC friends, they complain about how startups further on in the investment process are slow with providing information/answer simple questions that can save them in these internal VC grilling sessions. Startups take note of this easy win.