Given behaviour:

When tabing through objects in writer one by one object is selected. But the sequence is top down in the view and not in the document. So when having 2 pages visible besides each other an object in the top area of the page shown on the right side is marked prior to an object in the bottom area of the page shown on the left side although the sequence within the document is just the other way around.

Scenario:

Apply a format to each frame in the document. That for, you might have a frame template you apply to each frame. This in our case will shorten the document as the frame requests more space in page margin area instead of pushing other document content aside. So you mark a first frame and apply the template to it. Then you press tab. Now a frame in the midst of a page shown on the left is marked while there is an other frame a tiny bit below on the next page which is shown on the right.

When you apply the template now, the document gets shorter so the just reformatted frame now is a little bit below the frame on the following page, so that one will be skipped when pressing tab to mark the next object.