It is really quite simple - they get paid for each actual delivery - i.e. when a package actually leaves the delivery vehicle - with the appropriate signature (if required). If they go to a house 3 times, they only get paid for the time that the delivery was made, not all three times. Not to mention that house numbers are sometimes difficult to see - if they are there at all.
I feel for the spacemountainmom. It is a catch-22. You're damned if you do...damned if you don't. You get blasted by putting in the extra time (i.e. the Saturday's, ) because you've missed (I'm just guessing here) every family function, Christmas/Holiday program your children were in, plus your "other half" essentially becomes a sole parent. You also get blasted by the management for "wrecking" all those other family's holiday's, to finally enjoy some time with your own family, by not making deliveries on Christmas day.
In our area - there has also been snow and ice, which adds to the amount of time each delivery takes, because if people do not properly have their driveways and sidewalks shoveled and salted, it just takes longer to navigate that, to make sure you do not injure yourself on the job.
Many on-line companies seemed to be a bit overwhelmed with the response to their last-minute offers of guaranteed deliveries. There is no way to predict how many people waited until the last minute to shop.
Yes, I did some online shopping for Christmas this year, and all our on-line stuff was delivered before Thanksgiving. Christmas comes every year, there is no reason to wait until the last minute. There is no way I'd leave something to chance in December.