An officer being nice and saying you do not need to worry does not mean that it is the reality of the situation. Most of the time, they are just saying this to keep the situation calm and amiable it makes their jobs easier. Additionally, it is a similar case why they will not show you the results of your tests. If they did, drivers may ask to retake them or argue the results which would make their jobs more difficult or would not be allowed.
Regardless of what an officer says, once you blow at or over the legal limit, you will be arrested. Any niceties beyond that are their attempts to keep the situation deescalated and calm.