Trendline error

I have a problem with the trendline tool in excel 2010.

I fit perfectly a 5th order polynomial into my x-y scatter chart but then, when I use the equation supplied by excel for that specific trendline, the values obtained are way off (see attached file).

When I create a new chart with the equation given by excel, it is obviously completely different from the original one. And I need that equation to be correct because I want to use it for a model I'm developing in a FEM software.

As far as I know and have seen, this problem is might be associated with the Excel Tips "Polynomial regression. How can I fit my X, Y data to a polynomial using LINEST?", but I'm having some trouble in using that function.

Why a perfectly matching 5th order polynomial trendline gives an equation with an output as wrong as it is? How to explain such a deviation?

How to obtain a really representative equation using this tool?

Best regards,

Rui Pedro