Once in a blue moon?

The inexplicable rears its ugly, and unexpected, head.

Off and on all day "they" were telling us about this "Super-moon" which was, supposedly X% bigger than the norm due to the vagaries of its voyaging.

I didn't expect a lunar blip for at least 2 reasons
1. There'd be umpteen done by other folk.
2. The announcement of any manner of celestial happening virtually guarantees 9-10/10 cloud cover.
Tonight being no exception. Anyway I might as well have a poke at it.
Most of the early hours it was possible to point to a lighter part of the clouds and state, with reasonable certainty ... "That's where the moon is hiding.'
For a few microseconds it was in sight and I got a shot with the shiny, new, well-behaved 90-300mm.
By the time I'd swapped to 500mm mirror it was rapidly ducking for cover again.
I almost always shoot one, see what the camera has opted for, then start to mess around in manual mode. This time I didn't as I was changing to 500mm and it wasn't until I got it uploaded that I properly saw what I had.
ANY explanations gratefully received.