I can't say it any better than Judge Reinhardt, who authors the opinion:
"As an “attempt” in the state of California requires only “slight acts in furtherance of the [criminal] design,” People v. Superior Court, 157 P.3d 1017, 1022 (Cal. 2007) (emphasis added), one would reasonably expect that the California definition was categorically broader than the definition at common law, which requires a “substantial step towards committing the crime,” United States v. Sarbia, 367 F.3d 1079, 1085-86 (9th Cir. 2004) (emphasis added). Strangely enough, however, we are required by precedent to conclude that the two definitions are functionally equivalent."
Hence an attempted robbery in California entails a "crime of violence" for guideline purposes even though such an act is ostensibly impermissibly broader than the common law definition. The basic rationale being that even though the California Supreme Court has said that only a "slight" act is sufficient, as actually applied by the California judiciary, that's not in fact the case: they actually require a "substantial" act in practice even though precedent says that the test only requires a "slight" act.
You also gotta love Judge Reinhardt's concurrence to his own opinion. Take a look:
"It does not take a learned legal scholar to grasp the difference between the words “slight” and “substantial,” but it does, apparently, take members of the judiciary to equate the two terms. . . . “Slight” and “substantial” are opposites — according to the dictionary if not the Federal Reporter. “Slight” is defined as “small in amount, degree, etc.” or “of little importance . . . ; trivial.” E.g., Random House Dictionary of the English Language 1340 (1979). “Substantial,” by contrast, means “of ample or considerable amount, quantity, size, etc.” or “essential, material, or important.” Id. at 1418. Today we have compared a definition of attempt that requires an act “of little importance” with a definition of attempt that requires an act that is “essential, material, or important,” and we have held that the two standards are the same. That conclusion is nonsensical. It is not our function to devalue the English language by disregarding the meaning of words.
Nevertheless, because California’s “slight acts” test appears to be indistinguishable from the test employed in Nevada, I am bound by Sarbia and therefore compelled to reach the conclusion that I and my colleagues unanimously do. My obligation to follow precedent must override both logic and my sensibilities — not for the first time and, I am sure, not for the last."