Curve fitting with Excel and using resulting equation for subsequent calculation

I have a small data set consisting of less than 100 x,y pairs. x is a distance measurement horizontally from a wall along a laser level line. y is a distance measurement vertically from the horizontal down onto an imperfect brick lined concave shaped water ditch. That's the setup. So, the cross section is a shallow parabolic shape. What I need to do is determine the area inside the parabola as a function of height from the bottom of the bowl. The measurements are scattered, so after graphing the data in Excel I did a curve fit and had the equation show on the graph. Using that equation, I then generated a larger number of new x,y data points. This is so I could create a table of height vs. area to a finer resolution than the original data. BUT, the generated data set, when graphed, DID NOT line up with the curve fit (trendline). But it should, right?! Just to test that expectation, I made a trendline through the new generated data set graph and the equation from that turned out to be exactly the same as the fit equation from the original data. As it should, but again, the graph of it doesn't overlap the original. What the heck is going on? It's not even close but way off. For this job, I'll send my original data set. Please help figure out why this is coming out wrong and help get a fit equation that works. I imagine this would take a half hour for someone who is proficient with curve fitting and error analysis in Excel. Thank you, Gareth