Transaction Group _ndx Question

I created a Transaction Group to store cycle data to a table. My customer is querying this table to transfer the data to his system. He uses the _ndx column to detect new data. We are seeing this column reset to a lower number sometimes. It remains unique but is not always higher than a previous record.
How does the TG generate the _ndx data?

It's usually the database engine that does that automatically. (The INSERT query normally doesn't pass any data to that column.)

What engine are you using?