Single action lists and parallel projects do behave pretty similarly, but there are some differences, as you've noticed.
Single action lists have all of their available actions presented as if they were next actions, whereas parallel projects (or parallel action groups) show only the uppermost one in the list. You see this difference only when you've got the view bar set to show only next actions. Single action lists also style all of their actions differently by default so they can be readily identified in a commingled list. It is possible to configure single action list styling to style the actions like ordinary actions (see the Style preferences) but they will never be identified by next action styling, unlike ordinary actions. Or you could style them all like next actions, but then you lose the ability to discern which is which.
Single action lists are provided so that you can have a "catch-all" container for actions which might otherwise have little or no connection with each other, and don't merit project status. I put a Miscellaneous SAL in most of my big folders to capture the bits of flotsam that just need a single action.
Another use of single action lists is to handle areas of responsibility. The canonical GTD notion of a project is that there is a goal, and once you've reached that goal, the project is complete. Okay, that's great, and matches well with OmniFocus projects, but what about the many things in life which are ongoing responsibilities, perhaps with no defined endpoint? A project to maintain your health in 2011 seems a bit silly, as presumably it will also need maintenance in 2012. Similarly, who wants a "Take care of pets until dead" project? Wouldn't a single action list called "Pet care" be a more cheerful encounter? You can still have sequential groups of actions if you have multi-step tasks, and those groups can be set to repeat if appropriate. With the use of start dates, future actions can be hidden until ripe, so you don't have to skip over tasks to get the cat another rabies shot next year to get to the one reminding you to buy more cat food or clean the litter box.
Tasks in purple (with the default style settings) are next actions. If it is in a sequential project, it is the next thing to do to move the project to completion. If in a parallel project, there may be other actions which could also be done, but this is the first in the list, and OmniFocus presumes that it is first among equals and styles it as a next action. Next action styling is overridden by due soon or overdue styling.