First off, most people would definitely interpret "source" spells vs other spells differently, because the game doesn't explain much off the bat. It makes you think source spells are different.
In any case, you can easily explain this by saying the most powerful source skills are the ones that attract the voidwoken, and those are the ones that are being suppressed because they use a lot of source.
Therefore simple things like moving earth and generating some fire aren't enough to get on voidwoken radar and don't use that much source. Or more obviously the collar doesn't block every ounce of source. Who says the collars work perfectly? Also considering magisters and everyone else uses spells, they don't seem too worried about the source expended in them. but summoning meteor showers, divine blessings, thunderstorms.. etc are major enough to be felt through the veil.
