The plotting boards did not need to have highly accurate measurements, any more than your travel atlas.
Get me within 10 or 20or even 50 miles and it will be just fine.
So. to sum it up the carrier battles took place within the 10th parallel.
Wow.
Kind of lays waste to your observation.
Not at all.
Firstly, the lines of longitude are going to be off by a lot more than 20 miles, even 50 miles. Sadly, we can't really measure that because the wiki proposes zero maps with actual scale for the expanding distance between each line of latitude the further south you go beyond the equator. But we should be able to dispense with the notion that we're talking a small difference.
The massive six month Guadalcanal campaign, which comprised the two biggest carrier battles of the war - Eastern Solomons and Santa Cruz - was fought in this area over tens of thousands of square miles (yes, tens of thousands of square miles) in the area of the Solomon Islands and other archipelagos nearby.
Google tells us that the Solomon Islands are 666 miles south of the Equator. So, yes, a huge number of carrier fighters were flying around on the open ocean in the neighborhood of 10 to 12 (or more) lines of latitude south. At that point, according to the FE monopole map as displayed in the wiki, it's pretty clear the lines are quite divergent. If each line of longitude began (at the equator) at about 69 miles apart (something I assume no one disagrees with, FE or RE), then by the time those lines reach the Solomon Islands they're quite a bit off. The error is not going to be a few dozen miles apart, but over a hundred. This means death to the pilots. And that didn't happen.
Furthermore,
even if the discrepancy were only, say, 10 miles off, there's the fact that these planes sometimes failed to find their carriers and survive (a minority overall, to be sure), but this means that even with accurate plotting boards mistakes can happen. The visibility of the
scouting planes were only about 25 miles out on a clear day. Fighters, whose cockpit had different designs (for different needs) could see even less than that. A deviation of 10 miles, against a ship that is
also moving (carriers didn't sit still during any battles, but always moved), often meant the fighters had to circle around looking for their ships even with highly accurate plotting boards. An error of 10 miles likely translates into mistakes of dozens of miles off, which would have resulted in far more sea deaths than recorded. These planes had ranges in the hundreds of miles, but they had to go to the enemy, fight, then return, and didn't usually have the luxury of being able to spending another hour looking for their carrier group. They had to find it fast.
And finally, what about all the other battles in other places further or closer to the equator? There is no record or indication that planes were fitted with different plotting boards for each battle. In fact, carriers and their fighter pilots often didn't know where/when they would be fighting. The plotting boards had to work for any part of the ocean. So, are you arguing that the plotting boards were off by 10 in some places, 20 in others, and 50 in even others? If the pilots didn't even know the margin of error that sounds like a recipe for sea landing disasters almost universally.