Outlook has the concept of Categories. You may assign multiple categories to each task. In Google Tasks, you have Lists. A task can only be member of one single list. GogTasks synchronizes Outlook tasks with Google Tasks so that a Category equals a List. This poses a conflict since the rules are different on the two systems. The way GogTasks "solves" this is by simply selcting the first Category Outlook returns when asked for the list of categories for a task, as the List in Google. What is the first category? There is no way to know for certain. If a task categorized as "Green" and "Blue" on Outlook, you simply don't know if it will end up in the "Green" list or the "Blue" list at Google.
How do you adapt to this? There are two ways:
Given the example above, if "B" is set as the character prefix for preferred category, "Blue" would be the List the task would end up in at Google. A better example for real world use is if you use a category naming convention inspired by the Getting Things Done methodology. If '@' means a contextual category and "%" means project name, you can decided that you want your tasks at Google to be categorized by context by providing '@' as the preferred category prefix.