What's it mean when I charging my iPhone a black plug icon is in my power bar?

Okay so I just got home, my phone was dead so I decided to charge it. I looked at it and it said 40%. Usually when it's at that point in charging there's a lightning bolt on the power bar, on the screen… It's weird how it was saying it's done charging. I'm kinda scared.

I looked on Apple web site and found out the lighting means its charging and the black plug means its fully charged. Although if your seeing that and its not fully charged it could mean you have a bug. The page on the pdf is 35 out of 156.

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