One word for HPD drivers. When FGT goes a bit wide before turn in and going for apex it doesn't usually mean than it's giving room. It's much safer to pass exiting the corner if you haven't managed to get alongside under braking.
It cuts both ways, though - it often makes sense for a GT to start closing the door earlier than normal when an HPD is behind to show clear intent, especially where there's only a short braking zone and the LMP2 can't simply slice by with late braking. I know it costs a little time but it tends to help set the exit pass up because the HPD can take that later apex and tuck back inside and it ends up costing both drivers relatively little time and reduces the risk tremendously. Out of the slower corners the HPD just doesn't have the grunt to punch past the GTs down a short straight - too many GT drivers won't give up half a second or a second to facilitate a pass but expect a prototype to sit behind them and lose 3-5 seconds.
The performance differences are actually quite frustrating (if realistic, judging by the ALMS races) - in GTR2 the GT1s don't get held up too much by the GT2s in the corners, but fly by them down the straights, but here the LMP2s are barely any faster top end than the GTs, but massively quicker in the faster corners, leading to a lot more frustration and confusion (and danger!).