There's no reason to expect refrigerators of constant quality to get more expensive in real terms, and NPV calculations traditionally assume a reasonable (and positive) cost of capital, likely exceeding the rate of inflation.
So I would've said 1000 + 920 + 850 + 780, or something like that.
You mean no such reason that you want to find. Same extremely lowered (shit) quality things cost same present value, so increased through time with inflation, items made more complex as this is the case with fridges involving unnecessary 'engineering' and 'development', adding additional costs, old inventions hit wall for improvements after a while, probably we could find some more reasons if we analyse how prices increase for the same kind of product in time, which is the norm for things exist for multiple decades. (you may think about months or few years timespan here of new inventions/developments with maturing manufacturing and the pay off of investments of develping something new, but it is so very far from the situation here).