I could be wrong here but I think that the 405cx differs from the 305 in the way it calculates calories burned. I believe that the 305/405 use just your weight and distance but the cx involves heart rate in the equation which Garmin claim gives more accurate results.
If I use my 405cx without the heart rate monitor it claims I burn far more calories than when i run with it.
Having said that, I'm led to beleive that there's no real way to reliably monitor calories burned anyway.
Edited: 30/09/2011 at 23:29