Chart of Accounts Present Balances

[Mike C.]

My understanding is that the Present (USD) column on the Chart of
Accounts should list the balance of an asset account.  I have a number
of bond accounts that include a purchase transaction and a sale
transaction in the future when the bond matures.  Some of the accounts
list the purchase value of the bond and other accounts show a zero. 
When I run an Account Summary report it shows the correct value in all
cases.  As far as I can tell there are no differences in the way the
different accounts are entered in terms of commodity value, type
account, and all other parameters I can see.  It does not appear to be
related to the purchase or sale date.
Can anyone explain these differences and how to make it consistent for
all accounts?
Thanks,
Mike