At 21 weeks my son weighed 1lb 1oz so you are good. I actually posted a post just like this one and the app doesn't give a range but all the mommies who commented said the same thing and some where right on the same track as me so you're fine.
My baby should only be 7oz according to this app. It's not an exact science.
At 18 weeks mine was 9oz. My daughter was 1lb at 21 weeks.
At 20 weeks, my baby was 14oz. I'd say by now (23 weeks), she's at least what your baby weighs. The app isn't wrong, but every baby is different and the app would be better if it gave a healthy range of baby weight each week, like the bump's app does. If your doctor isn't worried, everything is fine :)
I am 22 weeks And last week my baby weighed 1pound :)
I went to my doctor at exactly 21 weeks and my baby weighs about 500 grams. Thats about 200 grams that the app says my baby is supposed to weigh. I just think the app is wrong, because my doctor didn't say anything.