Depends, the second one was a request for the site staff. The first one was simply because I was curious to see if HP 6 was already up online. When I saw it was, I've typed several other authors and saw most had part of their work online too. So I proposed the subject to the staff.
Before going freelance, I've worked 4 years in the same group than them. I know most of the site staff well enough for them to think of me when they have a subject fitting my capacities. It's a good co-operation.
No, but I usually do 3 to 4 for them and several others for other publications.
Roughly I write the equivalent of 30 to 40 pages by month. Currently finishing a 10 pages request for an DYI Computing magazine on storage device. Boring.