Travelling to the USA from the UK by Air Visa exception?

Just got back from the USA (California) travelled from LHR to SFO. We filled in our visa-exception forms online as we were required to do.

On reaching the USA customs we were asked to fill out green forms to accompany our passports.

Why did we have to fill these forms out with the same questions as we answered on line.

The green form goes to a different department and half of it stays in your passport till you leave

The same happened to me in North Carolina, I was told it was because the system has not caught up with the tecnology yet, but in time the online one will be the only one required. x

Indeed, this is a very good question. As I understand it, the on-line version will eventually be enough. It is pretty silly to fill out the information twice.