Dai, there have been many words written on this subject and as recently as last week Ernie posted in the Le Mans thread some wise words about the URD 992 and how quick it was in practice/Qualy but almost impossible to race for an hour.
As Casrten said above, it's a completely different set of variables in the race. You have other cars around you at the start (and hopefully during the race), therefore your braking and lines are more dependent on what others are doing in front, behind and beside you at any given time. I'm not sure if AC physics allow for reduced downforce when following another car but it certainly feels like it to me. Also, (and this might just be me), it often feels like different servers give a different feel. For example, you practice like a demon, get good times and set ups only for the race server to come up and the car feels completely different.
Add to this the possible heavier fuel load /tyre choice and the distractions of keeping an eye on your mirrors/Realtime gaps/ radio chat and you are sure to be slower in the race.
When I started down this slippery slope of Sim Racing five years ago I got frustrated by getting good lap times in practice only to fail to get anywhere near them in race trim. And shock horror
.......the news is it still happens after all this time!
Of course there are the Aliens who can replicate the times without even thinking about it but for me the important thing is to have a good race car, stable, reliable yet fast enough to keep in touch with my peers if possible.
So now longer do I get frustrated about my ability to get close to the fast guys in practice, but my measure of a successful race is if I can stay on the track, make minimal mistakes (of my own doing) and have fun racing someone. I do most of my practice in race trim now.
Last Wednesday I was running well in the Abarth at the Deutschlandring, consistent lap times (not the fastest by a long way) and heading for a steady fourth place overall. But halfway through the very last lap I momentary lost concentration and stuffed it in a ditch and couldn't extract it without teleporting to the pits. So all that effort for nothing...............
Lesson learnt!! (I was furious with myself for hours afterwards
).
As Steve said, being super quick is no good if you throw it at the scenery and lose all that time gained, it doesn't have to be 'tortoise and hare' but having a stable set up that you can race to the maximum of your ability will go a long way to a successful race day.
End of Sermon!!