How good is the yazio app and how can I use it to loose weight?

I don't know where else to post this, so here you go:

• How reliable is the yazio kcal counter
• Does it use anything
• Are there FOSS alternatives

the reason I'm asking is because I weighted everything I ate and put it into the app, according to the app I should have eaten 1600kcal, but I feel lile I ate 3000kcal

EDIT: I mean the kcal I ate all day, I didn't eat that much at once obviosly

would appreciate help and advise