A bunch of Kindle owners over on Reddit have reported that the Kindle they just got had over twice the expected storage. The devices are showing that they have 8GB of storage, and not the 4GB listed on the product page on Amazon.com.
No one knows why this happened, although it has been pointed out that the Kids Kindle ships with 8GB of storage, which could mean that Amazon is substituting one model for the other when one ran out.
On the other hand this could be Amazon deciding to upgrade the storage, only they forgot to edit the product listing. (It’s not the first time there was a goof on the website.)
Your guess is as good as mine. In any case, if you got a Kindle with the surprise extra storage, enjoy your good fortune!