The blood test history will report on the previous 5 tests before the test you are looking at. So 6 tests all in all. The comparative test looks at the current test plus the previous test in the system organized by date.
Let's say there are 3 tests in the system. If you want to compare the latest lab test against the 3rd test in the system by date, you can simply adjust the date so that the 3rd test becomes the 2nd and run the comparative report. Not ideal I know but a workaround nonetheless.