Why does our swim spa manual recommend no more than 40% softened water?

I've read through all the threads on soft water questions and didn't see an answer to this question. Our swim spa is fiberglass. I read in one thread that with fiberglass the calcium hardness is irrelevant and Recommended 100% soft to help keep ta, ph, and ch stable. We have hard water (7), but not bad iron problems. Our owners manual states using no more than 40% soft water. Any ideas why?
 
Tested my water today and here are the numbers:
Softened Not softened
ph. 7.5. 8.4
alk. 80 80
ch. 0 200

i read in another thread that heated water (we do 98 degrees) is best to have ch target of 120-150. Is that correct? My other question is would you recommend 100% unsoftened water or 40% softened with 60% unsoftened?
 
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