Could it be argued
that the USN's lower rounds per kill ratio was facilitated by the fact that their artillery was configured and concentrated for point defence ie a ship? .
USN claimed rounds per kill for heavy flak late in the war were more like 60-240, (daylight short range-night long range) not 500 or 1500, and the number one reason was widespread use of proximity fuzes from early 1943, those low late war numbers assumed proximity fuzes. The tactical situation of shipboard AA was more favorable in that the plane had to come to the gun, but OTOH those late war targets were typically relatively small and fast a/c moving in three dimensions, not large bombers flying at constant altitude.
In 1944 British 3.7" and US Army 90mm batteries in the UK also achieved impressive rounds per kill ratio v V1's when they used proximity fuzes, also in defense of Antwerp from V1's, albeit the V-1 also flew a relatively predictable path.
However, another major element of noise in rounds per kill numbers for AA batteries is the accuracy of the claims of a/c shot down, just like air-air stats. The degree of variance between claims and reality was itself highly variable.
For example I mentioned before the value of 3" AA in defending Corregidor in early 1942, forcing the Japanese bombers to fly high, and holing them frequently per Japanese accounts. But the rounds per kill claim in that case, 500, is based on claimed downing of 50+ Japanese bombers over the campaign, and the actual Japanese losses were far fewer than that.
Joe