Obviously local percentage and national percentage are different: it is one thing to be the only pair in a club to make 7NT and another thing to be the only pair in the country to do the same. Differences rarely exceed 10% and are usually less than 5%.
But I have noticed a curious phenomenon - it often happens that ALL pairs on one of the local lines do better at national level than local level and ALL pairs on the other local line do worse

The improvement averages up to +5% across the whole line, but is not uniform, so one pair might improve +4%, the next +7%, the next +2% and so on.
Does anyone have an explanation for this? If you play in similar events, does the same thing happen? Certainly it is no coincidence. I imagine it is a statistical curiosity rather than some kind of error, but there still must be some mathematical logic behind it.
It's a small club, we're usually talking about 6-7 pairs per line. One factor that might just have influence is that we don't assign pairs to lines in a truly random way and the stronger pairs tend to concentrate on NS - but this is not always so and in any case EW seems to "benefit" as often as NS. Another is that sometimes we play more boards than most other clubs in the country, and on those few extra boards it is easier to obtain an extreme national score, as relatively few clubs will play them - but this does not happen that often.