Why is my BAC 0.000?

I, female, Weight 140, 5’5”, don’t normally drink alcohol (I just don’t like the taste) Long story short, I happen to find a beer that I like because it didn’t taste like alcohol (Smirnoff,-ice,Green Apple). It’s my last night at home before heading back of to college so my mom made me a great dinner and I decided to have one bottle to go with it (other times I drink I go to sleep really easily and I have a long drive in the morning so that was the plan). We’re talking and having a good time and I guess The way I was acting was a tiny bit off, or my mom was just reading too much into it, and she said that I must be experiencing the effects of alcohol. I was like “no I would act this way normally” but she wouldn’t drop it so I went up to her room and I found a breathalyzer. Low and behold I try it and it comes up 0.000. Like I know it was only one beer but at my body size and gender it should definitely be detectable (and it wasn’t like I did the test a while after having dranket, and I drinking in the span of less than an hour). My mom‘s like no it must be broken and then she tried it (she had about a glass and a half of wine) and she clocked in at 0.032. So it’s definitely working. So why is it saying that I have nothing in my system?