Display problem with Running Summary field

[HazMatt]

In transactions.fp5, I have two unstored calculated number fields (their actual calculation doesn't matter here):

A) "posted_debit"

: "posted_credit"

I also have two Summary fields:

C) "running_total_posted_debit" = Total of posted_debit (running)

D) "running_total_posted_credit" = Total of posted_credit (running)

And one more calc number field:

E) "running_balance" Unstored, = running_total_posted_debit - running_total_posted_credit

When I view a list of records in transactions.fp5, the field "running_balance" displays a running total of debits - credits, just like expected. However, when I display the same list through a portal (set to show 25 rows) in a different database, "running_balance" displays the correct numbers in the first 26 rows only. If I increase or decrease the number of rows shown in my portal, the number displayed always equals that number + 1. Is this a bug in FileMaker?