I haven't run into this in person, but my dad worked for years at a BigCo engineering firm that had multiple rounds of layoffs, and they seemed to use a fourth strategy: try to get as many volunteers as possible by offering some severance packages, and just take whatever random assortment of people said yes. The theory was that that was the way to reduce headcount with the least impact on morale, since nobody was actually being fired. In practice some people were strongly hinted that it was in their best interests to take the package, though. It also tended to be de-facto biased towards getting rid of the older/more-expensive employees (who would just take early retirement) without overtly singling them out.
There can also be accounting considerations. Sometimes contractors are the first to go, other times they are last. Carrying a contractor frequently looks much cheaper than carrying an employee.
There are entire companies dedicated to executing (pardon the pun) layoffs "correctly" and there are as many ways to do it as there are managers, but they essentially fall into three categories: cost-based (get rid of the most cost), skills-based (get rid of the people who don't have skill X that we totally need) and performance-based (get rid of the people who suck). Most of what you're talking about sounds cost-based, in which case there's two philosophies: get rid of the most senior people who charge the most (hello, age discrimination lawsuits) and get rid of a lot of junior people whose productivity is lower and expect the senior people to pick up the slack.
And yes, I have had the privilege of working in a Fortune 500 company and helping put together a layoff list - worst job of my life.