The best case is that they are doing it to get an idea of whether they should be pursuing you or not (if prior). You should politely decline.
I can understand how some may be hesitant to offer this info up since it is considered sensitive in many cultures, but I have no problem doing so. I understand that there is no malicious or malevolent intent in their request for this info (at least, there rarely is...).
At my last job for every hour I worked over my standard 40 hours I got an extra hour of annual leave. Standard hours in my field tends to be at least 45/wk so when I switched jobs I quoted a figure that included the value of this extra leave.