It's a classic "annoyingly-misleading wise-sounding statement", because there's an interpretation - which I think is the intended one - which _is_ correct; "every day [that you do it] it gets easier [than it would have been, ceteris paribus, if you hadn't done it that day]". So, on that down day where you get what feels like a bad result, you still got a better result ("it was easier") than if you _hadn't_ have trained the day before. Your environmental factors were pulling you down, but your accumulated training counteracted that. Your bad days are better than they would otherwise have been.

But that's not as snappy for a cartoon monkey to say.