Your Dermo probably subscribed a topical flourouracil treatment because it will do a much more comprehensive job than just freezing the clinically obvious AK lesions. Your dermo probably thinks you also have a number of sub-clinical lesions that he cannot see to zap with liquid nitrogen.
Over the past 15 years or so, I have been prescribed Efudex (twice), Carac (currently), and Aldara/Imiquimod (once). I have a lot of recurring damage (forehead, cheeks, & nose) from years of too much sun abuse, both as a kid and an adult. Each time, a large number of areas react that had nothing obviously wrong prior to the start of treatment. I think each of these areas represent(ed) sub-clinical lesions that would have eventually required treatment.
My current treatment regime is prescribed for only two weeks, once a day. I plan to make my own decision after the second week if I am going to continue it for another week. I would much prefer to "bite the bullet" for another week or two and do a good job cleaning things up than having to do this again sooner than later. My face certainly looks frightening but I can deal with the discomfort.
I guess everything has some humor to it. It is also almost amusing to sit in a meeting at work and see people looking at the mess on my face but not have the guts to ask about it.