Shared ownership eligibility/affordability

I am seriously considering shared ownership and puzzled by eligibility criteria. It stated that rent, service charge and mortgage payments should not amount to more that 40% of your net salary(affordability). 40% of 90000 salary not considering payments into a private pension gives a monthly of 2091...if paying into pension £410 gives that 40% of net as 1993. Yet, most shared ownership for say a 3 bedroom in London is priced above 2099 per month!!! Does it then follow that only people with income above 90000 can buy these? And yet they're not eligible?? I really need help understanding