They could also be holding F seats for Federal Air Marshals, who pretty much always fly up front and in pairs. The FAM assignments are always changing based on daily threat assessments, so those seats could become free at the last minute before departure.

[Interesting note: An acquaintance who is a FAM told me he was once on a flight with 14 FAMs because of a known threat also aboard. I'll bet there was a whiny thread on FT shortly thereafter with someone complaining about not getting his upgrade!]