Excel 2007 (running in Vista) is not always displaying the correct equation
for trend lines added to Scatter Charts under certain circumstances.

If you add a trend line to a set of data in a Scatter Chart, select a
polynomial trend line, and then set (force) the intercept to a specified
value, then the equation displayed in the chart for the polynomial trend line
is sometimes incorrect. The coefficient on the highest-order term is not
always displayed properly. The coefficient that is displayed is the
coefficient that corresponds to the polynomial trend line when no intercept
is specified. All the other coefficients appear to be correct. The trend
line itself appears to be correct. Only the display of the equation is in
error.

Sometimes when this operation is first completed, the equation is correct,
but if one toggles between not selecting and selecting an intercept (or
changes the intercept), the equation will quit displaying properly.

This can be checked fairly easily by adding a polynomial trend line to some
data and then display the equation for the trendline. Then force an
intercept value that causes the trend line to change rather dramatically.
You will see that the coefficients for the trend line all change, except the
first one. If this does not occur the first time, then try toggling the
intercept selection several times, and you should begin to see the problem.