Generally your observation is correct. It shouldn't matter if you work multiple projects. However, not all tasks are well coded. On certain projects the check-pointing is not handled correctly.

When the manager decides to switch projects it pauses the task it is currently working on. When you resume that task, you go back to your most recent checkpoint. You can lose progress on your task when this happens, if the project doesn't write checkpoints as often as they are supposed to (YAFU and probably others).

The best setup for picking projects is to choose two. A primary at 100% resource share and a backup at 0% resource share. With this setup you run the primary constantly unless it runs out of tasks, then it switches to the backup.