Even with the same dose, there can be differences in sensitivity from person to person and day to day. On top of that the dose itself could have been different due to time in the lungs, actual vaporization, etc. Also, if the first couple folks did not vaporize every single mg, they may have left a few extra mg in the pipe for the last guy.
33mg was a good place to start. I've also read here that 20mg is recommended for newbies.
9/16/17 Edit: general consensus is that 20mg or lower is a good place to start. Sorry and my apologies.
After that I believe Terence McKena once said that if the first dose does not work, try a higher dose next time. Maybe something like 40mg which worked for me, but your first two friends should decide that for themselves.