Forms 10.1 Update 1

I'm playing with the out-of-the-box calculation functionality using tables.

The help files state that Forms uses the same OpenFormula standard as Excel.

In excel, if you want to copy a value from one cell to another, you can use a simple formula referencing the source cell, e.g. =A4

When I try this on a form table though, it behaves oddly and appends the value of the subsequent cells to all cells. For example, here's the first row:

When I entered 15 into the Sub-Total field, it correctly got copied to the Amount field. Great.

But here's the second row when I click Add line item:

OK, why did the value get copied into the Amount field in the second row?

When I enter 30 into the Sub-Total field, this happens:

Is there a way to specify in the formula that the value should only be taken from the specified field in the same row as the target field?

I looked at the documentation. Unfortunately, the examples provided seem very rudimentary and they are not helpful.