I don't recall the FBI ever expressing surprise about the purity. But a lot of other people were surprised by the purity because they were expecting something from some biological weapons program.
The attack anthrax didn't come from any biological weapons program. Such programs concentrate on quantity not quality/purity. They use industrialized processes where purity is a waste of time and money. Dropping a pound of powder that's only 10 percent spores on a soldier will kill him just as dead as an ounce of 100% pure spores, and the production costs are much less.
"PURE" spores come from a lab. Concentrations of pure anthrax spores are created every day suspended in liquids like distilled water so that they won't float around. It's less common to dry the spores into a powder, because dry spores will float around and are truly dangerous.
Someone said....It didn't have to be that good. (Pure)
So why was it???
The significant issue with this anthrax was not "purity" but SIZE the spores were milled to the perfect size for inhilation. Like Baby Bear they had to be juuust right not too small not too big.
There were only a few labs capable of that kind of milling and of that few only Iraq had the motivation to do something like this.