Why would she do that?

So my brother in law and I have always been close I knew him since he was a little kid he's couple years younger than me so anyway for his Christmas gift I got him a blue watch I can't afford anything else so it's the only thing I could get him at the moment. Me and his girlfriend went to the mall she was looking for something to buy for her mother she was looking for a watch to gift her mom for Christmas

I didn't wanna spoil the surprise for my brother in law so I didn't want to tell her what I got him but I did she suddenly picked up a watch and said she was going to get him a watch I begged her not to because she already got him so many gifts so I felt like if we both got him watches it would be pointless for me to give him the watch she said she would get him a watch for his birthday. Today he told me he already knew what I got for him so I asked her why she told him she said she gave him her watch for Christmas because she didn't think she would remember to give him for his birthday she said it with attitude too. I was confused on why she was acting this way she already bought him so many expensive gifts so I just watched to get him something nice because he got me a PS5 which is expensive I don't know what her issue is..

Why would she do that?
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