Why does my trendline equation change?

I've got a somewhat complicated XY Scatter chart. When I view one data series, the linear trendline equation is 'y=-30.179x+5427.5'. When I add a 2nd data series, it changes to 'y=95.117+331.52'. The 2nd trend line is correct.

Why would this happen? The data set for the 1st series has not. When I add a 3rd, 4th, ... 7th series, they too are messed up.

The data subset is generated by a VBA search macro. I have confirmed the data is not different. Anyone have any clues here? I have plotted the data on the scatter chart, then manually added the trend lines, with the same results.

Thanks for any help.