Chart showing incorrect plot wen opening some other file

Situation:
A single workbook containing charts is opened:
results look well

Now when I open a second workbook (alt+tab to folder, double click workbook) , the charts (in both workbooks) suddenly show the wrong results:
open1.PNGopen2.PNG
Plot contains trendline which fits the data and is displayed together with equation on the plot
(Upper image correct, lower image showing incorrect after opening other file)

It does not make a change when I select the view side by side option.


Please, can someone explain why does this happen and how I can fix this?

Thank you,
Mark