It’s not that they might be exaggerated they absolutely are, as would be the claimed ratio’s of any other country’s WWII fighters. The problem is, the degree of exaggeration is highly variable, even for the same plane used by different air arms or units, places, missions etc. And the nature of the opposition, not only quality of enemy fighters but how many of the opponents were fighters. So unfortunately those numbers are not anything solid, or meaningful IMO, except in very limited circumstances.
To give one example of meaningless, F4F 5.9:1 in 1942. That’s claimed against all opponents. But the natural inference is of some similar degree of success v fighter opponents. The actual kill ratio of the F4F v the Zero in 1942 can be counted fairly accurately, since relatively small scale combats, usually all F4F’s on one side all Zeroes on the other (among fighters), and both side’s records pretty complete for that period. It’s right around 1:1 give or take just a few a/c. That was a very good record by the F4F compared to other Allied fighters which saw significant combat v Zeroes in 1942, they were all distinctly on the short end of the stick. But how is 5.9 anything ‘solid’? (again not all that difference is exaggeration, a big portion of it is also F4F kills of Japanese non-fighters which rarely shot down F4F’s in return).
To give an example of limited usefulness, the USN calculated the kill ratio’s of the F6F and F4U in the period Sept 1944 to the end of the war v Japanese fighter types only, as both around 15:1. Besides exaggeration that included a lot of fighter types operating as kamikaze’s, but in that period it’s likely the proportion of fighter type kamikaze adversaries was also similar between the two US types. So in that case a claimed ratio might be useful to indicate lack of any big difference in F6F and F4U effectiveness v Japanese fighters. But for the whole war the claimed ratio’s of F6F and F4U’s reflect a difference in opposition (more non-fighters for carrier based F6F’s, weaker Japanese fighter units in 1944 than 1943 when F4U saw a lot of action in Solomons from land bases, etc) besides exaggeration, so aren’t comparable.
To a lesser degree such ratio’s also reflect differences and questions about which friendly a/c losses were counted as due to enemy a/c, and which a/c were even counted as lost (say among those returning to base damaged beyond repair, which itself depended in part how many replacement a/c were available).