Bookworm Deluxe wordlist not understood. Any insights?

Question

I am looking at the "wordlist.txt" file for Bookworm Deluxe (an old game by PopCap), and I have not been able to make sense of it. Most of the lines seem to be pieces of words, often with a single digit prefixing them. These numbers do not constantly represent a same letter combination, as 2rdvark would seem like 2 is "aa", but 2s is also in the list, and "aas" is not a word. Take note that the game has to have letter rarity information somewhere, but bonus words have their own separate file. Here is the file as a Gist.

Answer 1

This answer is based on this post on codegolf

It mentions

The rules for unpacking the dictionary are simple:

1. Read the number at the start of the line, and copy that many characters from the beginning of the previous word. (If there is no number, copy as many characters as you did last time.)

2. Append the following letters to the word.

It also mentions the word list with this output

aa
aah
aahed
aahing
aahs
aal
aaliis
aals
aardvark
aardvarks
aardwolf
aardwolves