Spring wound too tight

A home owner from Machesney Park, IL says:
In reference to my order (S160-9, 15-191 and 15-150S) I would like to give you the following feedback. The Pivot shoes and spring retainers were exact matches to the existing parts I was replacing. The two springs I ordered however, though both were marked the same as the existing broken ones (#9.0 35"), did not match each other. One slipped right on the retainer like it should, but the other one was wound much tighter and I had to unwind it and then wind it back on to the retainer. Because I had everything apart already, I went ahead and installed them but now have an unbalanced condition for opening and closing the window. My guess is that the spring wound tighter was either mismarked or wound on a smaller mandrel than it should have been. I have 9 windows like this and most likely will need to fix this one again and possibly others in the future. How can I be sure I will get properly wound matching springs with the same part number if I order again ?
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Tom from SWISCO responded:
That is odd. I'm not sure why that happened. How many coils are used on each side of the sash? Also, how heavy is the sash?
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