Luck.
We know the probability of finding shinies and what affects the rate, but in the end, it all boils down to luck. When you flip a coin, there's a 50% chance it'll be heads and 50% chance it'll be tails. If you flip it 10 times, statistically you're likely to see it land on either side half the time, but that doesn't mean it will turn out that way; it's completely possible it'll come up tails or heads for all ten flips.
Same with Shiny Hunting. If something's got a 1/100 chance of being shiny, it doesn't mean that on your 100th encounter, the pokemon will be shiny; just that you're likely to have encountered one by then. Since it's all luck, you could still go another 300 encounters before you finally see one.
So, you happened to get lucky with a few of your encounters and not so much during one of the events. Nothing more than that, really.