refinancing & net worth

[Danny Shore]

http://www.investopedia.com/articles/pf/07/refinancing_mortgage.asp#axzz2H8V7K2FB

I built an amortization table to corroborate his "net worth" numbers but there are 2 conceptual issues I need further elaboration on.

1. Why are the net worth savings based on the beginning of the period principal balances of the loans (i.e. $200,000 instead of $199,753.11)?

2. In the 238th month, why does net worth become negative? Is it because of the higher overall interest paid over the longer loan term?