Hmmm...well, in my experience (and this is PRE the recent US/Canadian Christmas issue - I drove last time) which is subject to change, the airport doesn't take as much time with each person because they have a deadline - planes have to take off and land eventually and holding one person up creates more chaos - where as at the border, if one lane is held up, vehicles choose another lane upon seeing the back up. Additionally, I think that the airport personnel feel more pressure because of this; they can hear comments and complaints. At the border crossing, the IO's are not subject to the same deluge; every other person wishing to pass is in their vehicle and, even with the windows down, they won't hear the same noise, confusion or frustration and so feel okay taking more time with each individual.
The requirements, at this time, focus on different things I think. At the land border, they seem more concerned about long term ramifications of letting you in (ie will you leave). At the airport, seem more focused on the short term ramifications of letting you board the plan. Slightly different agendas with the same purpose in mind.
Just my perception based on my experience - no yelling at me!
