I don't know if one is more accurate or not, but I can say that I try to look at as many as I can find when trying to figure out when I want to schedule my next trip. I tend to use them as "guides" though. I figure they can give you an approximation/best guess of the crowd levels (i.e. if all the calendars estimate a park's crowd level at around a 5 or 6, I'm guessing it is unlikely they will be closing the doors to the park due to max crowds.)
Most of the crowd calendars seem to group the various levels into 3 groups (notice the color codes for some of them for various level ranges.) For example, while
Undercover Tourist's calendar breaks out the range between 1-10, they also color code them to 3 "groupings", Lowest crowds, Average crowds, Highest crowds. Personally that is about as good an estimate as we can probably expect. And understand that "lowest crowds" may no longer mean the parks are empty, only that compared to the other times, the crowds are smaller.
For me, I don't even really need that level of accuracy for Universal. This is where staying onsite at a Universal Deluxe hotel to get EP is a game changer. At Universal, the crowd levels aren't as important as I have a golden ticket.
And even during more crowded times, I don't have to worry about lines nearly as much as someone does at Disney.
So for me, if most of the calendars have a Universal Park as a non-red day (i.e. not the most crowded days), then if you have EP, it will "feel" a lot less crowded to you.