Why are Transaction Numbers not sequential in Accounts Receivable?

Summary:

I recently logged a question in Cloud Customer Connect "Why are Transaction Numbers duplicating in Accounts Receivable?" Based on the feedback, I was able to establish that not all of the AR Transaction Types were assigned to the existing document sequence.

In Development, I end dated the existing sequence and created a new sequence and added all active Transaction Types. To ensure I did not duplicate any of the previously created Transaction Numbers in AR, I started the sequence from 1000000.

When testing in Development, all transaction imported into AR and captured in AR, had a Transaction Number greater than 1000000 and none of the numbers were duplicated but there were gaps in the numbers (refer attached). The Transactions Numbers seem to run sequentially, but then jump forward by a thousand. I expected the Transaction Numbers to start at 1000000 and then to increase sequentially (without gaps).