Probably that when you page-down (eg spacebar on FF) the paging doesn't take in to account the fixed position element and so you miss out on that section of content

     1
     2
     3
    [4========]

4 is hidden by a fixed footer. When it pages up the next page is:

     5
     6
     7
    [8========]

and you've missed out on all the section 4 and 8 content.

So it's rather the facility of skipping by screenfuls of content rather than the key itself.