Question about solar?

So I just got the Rangeman a couple of days ago. I'm new to the solar watches. I read the manual, and it has a battery meter that I can see. Out of the box, it was at level 2 battery, I spent the day outside yesterday and it went up to level 3. Now today, after work, it's back down to level 2. My question is is this normal? Why didn't it stay at level 3 longer? And am I worrying too much. Lol like how long do I REALLY have on level 2 battery?