Apollo 11's leap into the unknown - 13 Minutes to the Moon Season 1, Episode 8 - BBC World Service 

Probably it's more complicated than reported here. But your questions are totally valid, would like to get some answers.

@@kbabioch A lot of things said in this series don't make sense. And I'm not accusing the producers of knowingly deceiving us... but rather, better quality control.

Initially the full thrust of the descent engine is well below the level to deccelerate at all during descent, and it only gains that ability as fuel is burnt. Thus it must at first increase speed as it descends, before later slowing down. This was anticipated. I have read that it was lunar gravity variations, or mass-cons that contributed to an excess of speed above the expected. However during early P63 the engine is already at full thrust and can't supply any more of the needed thrust to bring the speed in line with expectations. The business about the pressurisation of the tunnel linking the command and lunar modules, is relevant only if it saved a bit of fuel ie. the lunar module then had an initial mass, prior to PDI, exceeding expectations. Note for P63 the mass ( and for that matter the centre of mass ) is estimated ( by dynamical response ) rather than known precisely. For instance the lunar module had probably another 30 seconds of fuel beyond what was quoted to the crew by Houston. The only way we'll ever know for sure is to go up there and measure the remaining fuel in the descent stage! In any event it turned out to be good luck as the target area was full of bloody boulders, so going long wasn't all bad.

Please note that Don Eyles ( the P63 programmer ) has written a book, called "Sunburst and Luminary", which deals with all of this.

Why?

​@@JohnM3665570Because he's obviously one of those "free thinkers". Move along, nothing to see here.

Yes you are