Stick up cam with Solar Panel. 100% day, but battery decreasing each night

I have the stick up cam and the solar panel. During the day time, the battery is always at 100%. So it looks like the solar panel is working to keep it charged. However, each night over the last week, I've noticed that the battery is decreasing by 1% when I check the battery level roughly around the same time every night. Example: last Sunday night at 10pm, the battery was 99%. Then back to 100% during the day. Then on Monday night at 10pm, the battery was 98%. Then back to 100% during the day. Then on Tues night at 10pm, the battery was at 97%. And 100% during the day and so on. Today after about a week, it is at 93% at night. Has anyone else experienced this? It's odd that at night it's decreasing by 1% a day cumulatively. But during the day, it always gets charged back up to 100%. I would assume that the battery would fall to the same percent every night after it gets charged back up to 100% during the day. Like, every night should be down to 95% and then back up to 100% during the day. Not 95% one night, then 94% the next, then 93% the next, etc. I have not changed any settings since last week when I first got the solar panel.