Data page scope "Thread" dose not work as expected

Per Pega document, "Thread" means contents of a data page are unique to a single case.

So, I think it should means, it will NOT refresh during life cycle of a case. But it's not, per below test (in Pega 8.6.6):

Step 1: I create a case. It's a temp case, no case ID at this moment. Data page content is A.

Step 2: I update data page content to B in another browser page by another user ID in dev studio.

  (actually I update the underlying data table which this data page is based on)

Step 3: continue step 1, I submit this case, case ID generated now, and I open an assignment of the case.  Then I found Data page content change to B!!!   This is unexpected, I expect the data page content remain unchanged in WHOLE case life cycle.

For info, if I change the date page scope to "Requestor" and repeat the test, then in above step 3, data page content still A, however, even I create another new case, it's still A. It's expected for Requestor scope definition.

So my question is that, the "Thread" scope seems mismatch the definition.