How accurate is the flightradar web?

Hi there.

I am just wondering how accurate flightradar web really is. I working home office and i am very close to the inbound traffic for FRA.

Watching flightradar and seeing/hearing what is happening over my head not always seems to match.

Sometimes I have planes visible that or not on the rader, and sometimes their position seems to be off.

Any thoughts?

Helmut