They edit the profile, and delete any changes after that, so their profile will jump on the top of the list in the area they are located at. This way they will have more profile visits and potentional customers.
Many thanks Bristolian, this never occured to me. How naive am I?
It does ring true, however. I've 40 or so on my hot list mainly Herts and Beds. One particular girl's availability changes day by day and she logs in daily (early) in particular to say when she's already fully booked. The remainder that log in for no apparent reason may well be aware of the possibility of which you speak. Alice in Wonderland in particular comes to mind.
Another feature is that two MF couples log in frequently, especially as the weekend approaches. I guess the old man works and weekends are his only chance for a mmf.