Is there an issue with scavenger vault frequency?

My past 12 vaults have all been scavenger vaults, is there an issue with the chances? Under the 20% listed on the wiki it would be nearly 1 in 250 million of getting 12 scavenger vaults in a row. Is there any way I can fix this, or.am I just insanely unlucky? Scavenger vaults are awful, you can never get everything in time, even if I am speeding around with candy bars.