The smiley face on the home screen is based on both pollen data and your allergy log entries of how you feel. This means that the smiley face tells you that you may perceive high pollen levels as lower if previously, you logged that you feel little or no discomfort during similar pollen values.
So, if you do not log a response to tree pollen when levels are high, the smiley face will tell you that your perception may be different from the actual pollen score.
But suppose you are responding to grass pollen and you tell the app that you feel strongly affected by the high grass pollen levels. Well, the smiley will adapt to that.
If you know your specific pollen allergy it's a good idea to check the pollen forecast as well.