I'd think it'd have to do w/ confidence, insurance and strategy...

In a 30 min main if you can get 10 minute pit stops than in theory you'd only have to pit twice. However being that close to being out of fuel means that your 10 minutes better hit right when you're approaching pit lane or else you risk running out of fuel say 1/2 way thru a lap. So for a bit of wiggle room your gonna pit at less than 10 minutes which means now you must pit 3 times instead of 2. 3 pits at 8 min intervals means you finish the race w/ "2 min left" of fuel based on your strategy (pits at 8min, 16min, 24 min.....next at 32 but race is over at 30, therefore 2 min extra). So pitting at 8 gives you confidence that you can run as hard as you want w/o worrying about running out or leaning out. Have you ever pushed right to the brink on your fuel level....temps start to rise as your leaner, the rpms stay up while in flight, basically it makes your engine performance change and now you have to drive differently to compensate......my 2 cents