I suspect they aren't selling all the seats and leaving some blocked off.
Which is exactly what a responsible carrier should do.
In the bad old days, in a UK airline that shall remain nameless, we got round this problem by declaring the en-route diversion as the destination, and the real destination as the diversion, and then diverting when overhead the "destination" to the diversion/real destination, so long as that was open and looked like staying that way for an hour or so. That gave us 6 more seats to sell on a Viscount to, say, Majorca, and thus a profit on the flight. The FOI was kept quiet by, ummm........, little favours. I rather hope that in these enlightened times such creative flight planning would not be allowed.