The point is not really to slow down for it but to just hope you get lucky. I too brace myself every time, lol. Darkdrium has himself given perhaps the best reason why this is not a synchronisation issue, namely that mines work 100% of the time. But I have a couple of others as well.

Bombs are almost always found to be dropped in logical places, e.g. on the fastest or most obvious racing line. If you change course and fly near the wall you will often still cause the bomb to blow up and slow you down. Why would someone have dropped a bomb right next to the wall? Also, it not only seems highly unlikely that the player has dropped the bomb in a very strange place, but that that strange place just happens to be exactly where you drive to avoid the 'fake' bomb. It happens too often for it to simply be a case of you coincidentally driving into the invisible, 'real' bomb.

The other point is that if the bomb functions how you propose it does, it has very little to differentiate it from the mines. It's just a big mine that stands on it's own. I think more thought went into it than that, and they decided to provide a chance of its not going off, which they balanced by making the bomb the weapon that most slows you down (except for being hit by all three rockets at once or all five mines at once), and by giving it varying detonation proximities.

I might be wrong on exactly how the bomb works, but I do think its behaviour is intentional.