There may be a good reason for this.

The guys white slipping may have already gotten a GS. Since you don't get GS #2 unless there is no one else in the category who even wants GS #1, perhaps the thinking is "I might as well get some hours via WS, since I won't be getting a GS (since I already got one)."

In fact back in our line of time bidding days, I flew with a very senior guy who would use a bidding technique (not available under the PBS system we have now) to start the month with zero hours. Then he would get two green slips (prior to C2K pilots got two GS in seniority order before a junior guy got even one). Then after that he would do nothing but WS easy one or two day trips to fill up the rest of the month. Remember that for a regular line holder a GS pays double ONLY if the rest of your month's flying equals the GS threshold, which ranges from 72-75 hours.