Breadth is *equal* weight
Spx is *market* cap weighted

ie. Calculating breadth $AAPL $AMZN $Msft weight as much as $coty $ua $nws

However, amzn+aapl+Msft weights 16.4% vs coty ua nws 2.5%

If amzn+msft+aapl up but coty ua nws down : breadth =0, but spx strongly up