One possibility is that the drivers in the program so far are OK. The companies in charge may be well acquainted with the initial drivers. That alone would not guarantee that all future drivers in the program will be safe. I don't trust the Mexican government's standards.
What is the average amount of accidents for the number of driver-miles over that time period for the pilot program? I don't know. If that number is very low, it may be too early to evaluate.
Another possibility is the program could have intentionally picked out very safe routes for the trucks. That would tend to make their US driving records safer when compared to all US drivers. For example, do they travel on California highway 152 (shortcut between interstate 5 and San Jose)? I don't think it's the most dangerous in the USA, but I have been on it twice and saw accidents both times.
Since I don't know what routes they drive, I don't know whether that is the case. Even if it were the case, it would not condemn the program. The point is just that some statistics are misleading.
What is the average amount of accidents expected, based on US driver statistics, for the same number of driver-miles over that time period in the pilot program?