That's got Lonsdale girth straps (long) with a dressage girth. With standard girth tabs the buckles would normally sit under the flap.
The buckles need to sit (a) where you can get at them mounted (b) where they don't interfere with your leg/foot/stirrup and (c) where they are most comfortable for the horse eg. not right on the edge of a pad which might cause a pressure-rub.
There's no 'standard' position, just what's most comfortable for the individual horse and rider.