Look in Core Temp, and see what it says Tjunction is..if it says 85c, then it's accurate, if it says 100C then it's actualy showing your temps 15c higher than they actualy are. Speedfan would be affected in the same way.
On my quad core, speedfan reports my temps 15c lower than they actualy are. Speedfan read the Tjunction at 85c when it's actualy 100c. The way it measures the core temps, is by measuring the differance from Tjunction. Tjunction - differance to Tjunction = core temp. For example right now I am reading 44c idle in core temp, which reads my Tjunction correctly at 100c. Speedfan reads my Tjunction at 85c(which is correct for dual cores, but wrong for quad) so speedfan is showing my temps as 29c.