Many large brokers advertise coverage far above the sipc-coverage-limits, often described in the hundreds of millions. That extra layer is a commercial insurance policy, usually written through Lloyd's syndicates, and it only pays after SIPC protection is exhausted in an actual liquidation.
Read the structure before taking comfort from the number. There is normally a per-customer limit and a much smaller aggregate limit for the whole firm. If a very large broker failed with a systemic shortfall, the aggregate cap could be reached and customers would share it pro rata.
Like SIPC itself, it insures against missing property, not against losing money in the market. A broker with enormous excess coverage gives you nothing at all if your trades go badly.
Related: sipc, sipc-coverage-limits