I would hazard a guess that Flash Lotteries, unannounced lotteries, lotteries posted at odd times, and lotteries lasting different lengths of time are utterly and completely intentional, to provide an opportunity to a wider variety of people to enter and have a chance of winning ... something.
Personally I like the variety, and sometimes getting a surprise when I click the lottery button. It's ..... unboring. And unlike some, I never crab about freebies. I simply appreciate them and express my gratitude. Some lotteries I will get a chance to enter and perhaps win, and some I will miss. Que sera, sera.