For the 2 soaring flights the form states:
..... The evidence must be by a barograph trace to the satisfaction of an official observer or alternatively may be by continuous visual observation.
Since most people working up to Bronze have a little way to go before being allowed X-Country many of the flights are certified by observation.
The other point to note is that the Bronze is a BGA, not an FAI, award so you don't need an official observer - instructors can also certify the flight.
OC619.