The idea is that less people take it in March
How does less people equal "hasher curve?"
Less could mean only the people who did poorly before and are taking the SAT one last time their Junior year in an attempt to boost their score.
Therefore March would mean an "easier curve."
As I said, there is no proof that one month's test is any harder than any other month.
And, as also mentioned, thinking such thoughts leads to defeatism.