Summary Lines are not exactly as most people think. They do not always summarize duration (actually rarely do they) and they should not be used for linking (predecessors/successors), assigning resources or for assigning work. A summary line is assigned its start date with the earliest start date of any task in its grouping. Then Project assigns the latest finish date in its grouping as the finish date for the summary line. The duration shown for the summary line is the working days between these start and finish dates, not the summation of the durations of the tasks in the grouping. But that’s not the issue you are addressing – notice that you can create a summary line (call it Pets). Now create a task (call it task Cats) to start today and run for 20 days. Then create another task (Call it Dogs) starting 2 months after the finish of Cats, also running for 20 days. Assign the resource Cat to the Cats tasks and Dog to the Dogs task. Now assign Pets to the Pets summary line. Go to the Resource Usage view. You will notice that Pets has resources assigned during the void time frame between the two tasks. That is only one of the reasons you should not assign resources to summary lines.
Notice that I do not use the term Summary Task (even though Project does). They are not Tasks” – they summarize and group. Summary lines group tasks, show the total time span of the group and sum up work effort of the tasks in the group. That’s all you should use summary lines for. Often if you assign dependencies to summary lines, you create false critical paths and restrict Project from keeping an accurate dynamic view of your project.