Firebase query with queryOrderedByKey().queryLimitedToLast(10) does not return one of the last 10 items - ios

Question

I am trying to load the last 10 products that have been added to the all_products table in firebase.
I run the following query with version 3.2.0 of the firebase SDK:
let fbRef = FIRDatabase.database().reference().child("products_all").queryOrderedByKey().queryLimitedToLast(10).observeSingleEventOfType(.Value, withBlock: { snapshot in
for item in snapshot.children {
let keyString = (item as! FIRDataSnapshot).key
However, the items that get returned are not the last 10 items in this table. I have about 1017 products in this table and the first item that gets returned in this query is item 973, while it should be item 1008.
My firebase looks like this (I used childByAutoId to add the products):
{
"products_all" : {
"-KFOiZDrbR4eq4fXSdMZ" : {
"affiliate_url" : "https://some.link.com",
"description" : "product description",
"image_url" : "https://image.url.com/firstImage.jpg",
"images" : [ "https://image.url.com/firstImage.jpg", "https://image.url.com/secondImage.jpg" ],
"merchant" : "merchant.com",
"price_usd" : "100.00",
"title" : "product title"
},
...,
"-KFtOmT_Hq-48chbHh7I" : {
"affiliate_url" : "https://another.link.com",
"description" : "another product description",
"image_url" : "https://image.link.com/anotherImage.jpg",
"images" : [ "https://image.link.com/anotherImage.jpg" ],
"merchant" : "anothermerchant.com",
"original_currency" : "GBP",
"original_price" : "255",
"price_usd" : "364.65",
"product_category" : "Bracelet",
"external_product_id" : "Anothermerchant-123",
"title" : "another product title"
},
...,
"-KFtOmTlcb5-PTQ6kTv8" : {
"affiliate_url" : "https://last.product.com",
"description" : "last product description",
"image_url" : "https://last.product.com/image.jpg",
"images" : [ "https://last.product.com/image.jpg" ],
"merchant" : "anothermerchant.com",
"original_currency" : "GBP",
"original_price" : "179",
"price_usd" : "268.5",
"product_category" : "Bracelet",
"scraped_product_id" : "anothermerchant-493",
"title" : "last product title"
}
}
}
EDIT: Also, the last element in the batch of 10 is number 992 - so the 10 items are neither in the last 10, nor are their all 'neighbours'.
First keys that were included:
-KFtOmT_Hq-48chbHh7I
-KFtOmT_Hq-48chbHh7J
-KFtOmT_Hq-48chbHh7K
Subsequent keys that were skipped:
-KFtOmT0Qj3mrCTlxOfj
-KFtOmT1jouJgm8x_Mi7
-KFtOmT4GUpXtd9Y90sq
-KFtOmT4GUpXtd9Y90sr
-KFtOmT57vidxDl_jpj_
-KFtOmT57vidxDl_jpja
-KFtOmT9yuy-1G8WBrsE
Then some keys were included and others again excluded, the 'coverage' is patchy.