This is by design, test cases and test runs/results are not the same concept in TFS. When you link the same test case in multiple test suites a separate test run an thus test result will be created for the test in that specific suite.
For test plans this works the same way, multiple test plans that reference the same test case in one of there suites will have seperate and possibly different results for that test case.
When you execute a test case the steps (and other information) are taken form the referenced test case and a test result is created with that information in the specific suite you placed it in. The results are not related to any other runs that have been executed referencing the test case other then runs in the exact same suite.
As you can see in the above picture separate results exist within each test suite, in different test plans, but also within the same plan*.
This may be a bit confusing at first because all results in all suites point to the same test case work item. But you could also ask yourself why you have the test case in suite A and in suite B within the same test plan if you expect the results to be the same.
*Note that this is not the exact way the information is stored in the TFS databases, but I went with for a simple representation to illustrate the behavior of TFS.