Well... good flowering nutrients should also contain nitrogen. Remember, potassium is a building block for generating plant material too.
Also, I think a lot of this is personal preference, or growing habit. Some do feed with only vegetative growth based nutrients the entire grow.
To answer the question: since there is a limit to what these plants can take in, it's all about the proportions. As they take up more phosphorus and potassium, this leaves less room for nitrogen.
Personally, I keep my flowering girls on vegetative growth based nutrients until they stop stretching, and I don't stop, I just cut back a bit as I make room for flowering based nutrients.