Question

How can h2 displace pb from pbo when it is lower than pb?

Answer 1

The activity series you're referring to is constructed for ambient conditions where the temperature is 25°C and water is a liquid. But, of course, thermodynamic properties can depend on temperature and especially on phase. At higher temperatures where water is ordinarily a gas, it has a lot more entropy than the liquid. That makes oxidation of hydrogen to water more thermodynamically favorable than what appears in your ambient temperature series.

For high temperature reactions where water mixes with hydrogen in the gas phase, the entropy effect pushes hydrogen up the series so it roughly matches iron