Ok, got it, quote from :
"Because these TV modes only depend on the TV encoder and the TV standard, TV modes do not go through normal mode validation. The X configuration options HorizSync and VertRefresh are not used for TV mode validation.
Additionally, the NVIDIA driver contains a hardcoded list of mode sizes that it can drive for each combination of TV encoder and TV standard. Therefore, custom modelines in your X configuration file are ignored for TVs."
So the predefined mode that NVIDIA chooses automatically cannot be overridden