stupid question.

would the fan and heatsink not properly seated on cpu cause false heat readings for cpu in bios and cause the pc to shut almost immiediatly.

I should imagine so, the CPU is maybe overheating.

why does the fan and heatsink not seat properly?

if your having heat problems the chances are you may need to take a look at your thermol paste, to much thermol paste is just as bad as not enough, please take care in this next step, remove the heat sink, clean off all paste and apply new paste, you only need a thin layer just enought for it to hold your heatsink to the CPU, if you put to much on the heatsink will not pick up the heat and you will get overheating.......
also make sure you haven't got the cpu settings to high in your cmos setting...