Overflow tank fluid boiling

Yesterday driving sisters car it overheated and the rad was about ~3/4 gallon short of being full. Added water and it seemed to be OK. Today seemed to be OK while driving but while stopped I saw steam out of the hood -yet engine temp was OK. Rad fluid was boiling in overflow. Let cool down and repeated same; boiling fulid in overflow but engine temp is fine which stumps me. Outside temp is about 75 degrees fahrenheit. Thinking this is a pressure thing rather than temp but this is outside of anything I have seen before. Thoughts?