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He understood that he didn't have the strength to hit it over the fence with a relaxed swing. He got to Fenway and, in the course of a week, realized that he could hit HR's without muscling up.
Think if you ask any athlete here, they'll tell you that the effects of a given playing environment can warp your game within 10 minutes, much less within a month...
Sexson and Beltre had their games warped by the sight of Safeco's LF fence. It conditioned them (and many others, such as Johjima, Lopez, Cameron, Gutierrez, etc etc). Take the fence away and they can resume their normal games.
We are not talking about saber theory here. This has nothing to do with math. The playing conditions affect your game, period.