Its possible someone added water holes to block deck. (made it like 55-59 deck). This would allow a great bit of water to shortcut to the crossover and right back to the radiator. Water in radiator or crossover would be significantly cooler than water in rest of block.
Its also possible something else is allowing too much water to shortcut back to radiator.
Study this link showing differences in 55-59 reverse cool system vs the 1960(and up) "conventional" cooling. Includes plenty of good pics and info showing differences.
http://www.pontiacsafari.com/EngineCooling/index.htm