This has been an issue for a very long time. Here's the answer, the way I understand it. The number of sufficient reserves is a moving target. A few years ago, some bright boy in the basement of the hive brain developed a statistical analysis algorithm to determine the probability on any given day of calling in a draft or volunteer. Seeing as the co would much rather pay 4:30 for a trip, vice 6 or 9, if the probability, based on an analysis of 6 months of historical data, is >0. You don't get to move the R-day.

Now, this might not be entirely fair but, WTH, the previous agreement (written before the algorithm) stated that: "11. Moving an R-day(s) A pilot may request to move an R-day(s) within a bid period. The request shall not be unreasonably denied." After the program became active, when pressed on the matter and the apparent abrogation, a member of the SIG stated that, the co considers increasing reserve utilization to be a justifiable reason to deny the move. People in the union have been aware of the situation for YEARS. When the opportunity to address it was available, ie during the last negotiations, other considerations rated higher priority, like fixing the MD-11 rigs. It appears that what the company had imposed unchallenged has been formalized and institutionalized.

This also might not be entirely fair but, it appears to me, that when setting priorities, the union considered the old adage, "You only have to be junior once."