Do you remember how snowflakes were distributed? The ones around your city I mean.
It was something like
10/14 gave 1
3/14 gave 2
1/14 gave 3
As a shot in the dark I could see them using the same formula for chests
I'm almost certain that they play with this. It's quite common for them to keep dropping in 3s on the last day of an event, but hardly at all at other times.
I've spoken to at least more than 10 people this week who have said that on Live servers this week purple and gold chests have been falling all over the place. I myself, in one of my cities, got 2 purples and 2 golds on the first floor alone... and yet on Beta it's been the same old - On Beta I'm now partway through the third floor and have had nothing except for 2 blue chests. My first thought is that they are pre-empting another wave of people complaining about Spire sets not being completed by giving our a glut of opportunities in the final week.
All things being equal though, and assuming you get 1 gold and 1 purple each week (possible, but not certain), then on average you could be approaching 1 artefact a week. So, of course there are naturally some people who will end up with 12 and some people that get none - Note, I went through a spell of 45 weeks in my Live city winning only 1 Magic Residence from the spire. That's an edge case. I think the maths said that the odds of that run happening are less than 1%. There will be many disappointed people though.
Interesting that they have said that it "could be extended". I think they are going to look at how many people are reaching the top of the spire and how many artefacts they win. If that's right then it's heartening to know that perhaps they do want everyone to get some.