On bag rolls, if you fail to get a bag when taking a zone, you get a bonus chance of a bag in the next zone. This keeps stacking up across the week. It is also tracked individually for different types of bags.
You'll see a message after taking a zone along the lines of
"You didn't win a bag this zone.
Your contribution for a gold bag is increased by 174
Your contribution for a purple bag is increased by 572
Your contribution for a blue bag is increased by 388"
etc
When you do win a bag, your stacked contribution for that colour AND BELOW is reset, but not above.
So, in your scenario, i can see you stacking up contribution across the week with your unlucky rolls on bags. Then you won a purple, and used up that stack but kept your gold stack. Then you won a gold bag with your big stacked contribution. Then, you simply got lucky on the next gold bag (which does happen).
This was implemented to combat the pure RNG on bag rolls. Basically, if you take enough keeps, you are highly likely to get bags.
On zone take exp / rr, check the home page of the website for latest patch notes. They recently changed it, so you can now see your contribution. Losers get their rewards halved compared to winners of a zone.
Max rewards for zone lock should be:
80 000 XP
20 000 Renown
4000 Influence
60 crests.
Max rewards for keep take should be:
30 000 XP
10 000 Renown
1200 Influence
20 Crests
I'm assuming rested XP / renown pots / other multipliers have an effect, but i don't know for certain.