Hi,
Because those 50, 51 etc. are done with very easy conditions of play (some tournaments are like that, but not that much). Compared to major tournaments, those "easy" tournaments have shorter tee distance (around 100yds shorter), low wind (5-10mph compared to 20-30mph during the last major) and rather slow greens (very fast greens compared to championship greens). With harder conditions of play, the scores are higher for everybody.
Regards