its just because they are different sizes, pumping at 100psi sounds like more, but if it has a smaller outlet then its going to pump less water than a pump pumping at 100 with a larger outlet, its like if there was a 100 psi pump the size of a houshold bin, its going to pump more water than a 100psi pump like the sureflow,
if the 60 psi pump has a higher flow rate, then its pumping the water faster, but at a lower pressure.
a 100 psi pump can produce that pressure at a lower speed because of its size. The 60psi pump produces 60psi but it has to run faster to get there, and as the outlets are the same size the smaller diaphram in the pump has to run faster.