Yeah, I see what WSB (Washington Service Bureau - the source of the numbers) is doing. They are basing it on when the transactions were reported to the SEC, not when they actually occurred. In that case, April is the lowest month for officers buying that I have data for (going back to early 2001), and could well be the lowest in a good deal longer than that.
I'm surprised that WSB would make an error like that, as they are the authority on insider trading. It really doesn't matter a whole lot, though. In April, there was very little buying and a lot of selling.
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