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Update:
I was able to fix the issue by changing option.

How you are related to your family member?

Change from grand child to parents
 
I guess changing from grand child to parents may not be correct, as it is from the applicant's point of view.
The form also provides an example: "If your parent lives in Canada you should choose 'They are my parent or step-parent'".

Even I faced the issue of $170 as Bio metrics fee while applying for 1-person. However, when I changed the "documents to prove Child / Grand Child meets LICO" from multiple documents checklist to just 1 single document (NOA), the Bio metrics fee was updated to $85
 
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Update:
I was able to fix the issue by changing option.

How you are related to your family member?

Change from grand child to parents

That may amount to misrepresentation. Secondly, if one wants to apply for a supervisa - changing from "Child Grandchild" to "Parent/ GrandParents" or any other options - makes the Supervisa eligibility vanish from the available options.
 
I guess changing from grand child to parents may not be correct, as it is from the applicant's point of view.
The form also provides an example: "If your parent lives in Canada you should choose 'They are my parent or step-parent'".

Even I faced the issue of $170 as Bio metrics fee while applying for 1-person. However, when I changed the "documents to prove Child / Grand Child meets LICO" from multiple documents checklist to just 1 single document (NOA), the Bio metrics fee was updated to $85

This one is an excellent observation. I can confirm this is exactly the glitch in IRCC website fee calculation.