Luckily for you Bo, I am able to answer your question(s).
Defensive assists are no longer an officially recorded stat, however, whoever is processing the combined stats, has obviously left it on there, despite them not being recorded in the games played.
A defensive assist is (was) recorded when a player takes a charge, and various other things that directly created a turnover, without being an actual steal. There has been a change recently in the interpretation of a steal, which has encompassed some defensive plays that were previously rewarded with a defensive assist. Now you are credited with a steal if it's deemed that your actions led directly to a turnover, regardless of whether the ball is kept in play. Taking a charge, however, gets no credit anywhere on a box score these days.
I have no idea how the top 10 are picked, though. The program must assign it somehow?
I can tell you one small tid bit on defensive assists though. Andrew Vlahov, in his first season of NBL after a stint at Stanford, led the league in defensive assists in 1991. I hope this allows you to sleep a little better tonight, Bo.