Why is deltaTime different in BP and C++?

Generally you should trust delta time… what why there no documentation about it, all you can do is study source code, this is main code for ticking:

https://github.com/EpicGames/UnrealEngine/blob/c9f4efe690de8b3b72a11223865c623ca0ee7086/Engine/Source/Runtime/Engine/Private/LevelTick.cpp

If you want 100% to have something in sync, then make multiple object rely on one object data

Also considering you freezing code, everything will freeze in that 400ms or 125ms point diffrence