Why does my girlfriend leave her phone face down?

So I have been seeing my girlfriend now for about 10 months and everything is great when we are together. We live in different cities and see each other maybe twice a week. We are very much in love and plan on living together down the line. Lately something has been bothering me, she has started leaving her phone face down which she never used to do. I casually asked about it one day and she said she likes the look of the new phone case. Thing is, she regularly checks her phone in front of me and I even know her pin (but I would never snoop through another person's phone no matter what). I know a lot of you will say its suspicious behaviour all of a sudden but I do really trust her and just want to know is this normal?
Why does my girlfriend leave her phone face down?
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