The key is in the underlying components

The market-moving aspect of the ISM manufacturing report is not its “headline number,” which tends to remain relatively stable. (In fact, it has been above 50 every month since July 2003). Rather, traders and analysts pay the most attention to two components — the “prices paid” and employment numbers — which provide a first look at how the CPI and non-farm payroll figures, which are released later in the month, might turn out.

Although much of U.S. employment is in services, the employment and prices-paid components of the non-manufacturing ISM index (which is released the third business day of every month) is watched as more of a confirming indicator of the ISM manufacturing report.