Am I confused as to what report variables do?

I have a variable, "totamt". The initial value is 0, the value to store is "amount", the calculation is sum, and it releases at the end of the report.

I thought this would give me the same result as a field at the end of report with the expression "amount" and the calculation of sum.

But instead it is giving me a huge number and I don't know where it's getting it from.

So how do I get a report variable to give me the total amount?

Thanks,

Michelle