So much this.
There have been massive SERP changes/fluctuations since September/October of 2015. For all we know, Google has been silently rolling out the new Penguin algorithm all this time. It is in their best interest not to announce its rollout when it is actually happening and instead wait for it to roll out, the SERPs to stabilize and then "announce" that they have implemented it. This way they get to avoid the massive PR sh*tstorm which always takes place when they announce large algo updates the moment they take place. Why do you think Matt Cutts is no longer the spokesperson for these updates? They learned that it is better to just be as vague as possible and/or keep everyone in the dark when it comes to large updates.