The Resolve scopes show the full range signal. The scaling to video levels is done on the Decklink output when you are monitoring, or laying off to a tape.
The only way to see video levels on the Resolve scopes would be to use a 1-D Display LUT which would do full range to video level scaling. If you are using such a LUT, please make sure you set Decklink monitoring to full range.
Of course, if you are using Ultrascope for external video monitoring, you would see video levels since it will look at the Decklink output.