It's pretty self-explanatory: Once you fill up your meter of defense/resist/attack, it won't show any more that you may obtain, however, it's still there. One of the most common problems this has caused would be Leviathan VS CIV: Both show the same attack power, a full meter - However, Leviathan is MUCH more powerful than Cold Iron.
On resists, this is also quite a problem - If you have any resist variant or pendant, you won't be able to see it on the overall status because of the meter limit. And on shields, we can't even know if an one star shield offers the same resist as a five star, since both show the full meter. It might just be more than that. Variants on shields aswell, end up being a bigger problem.
And considering resists only hit their cap at two full meters, (...although the most you can ever get is four meters. Silly resistance system.) it ends up being a game of guessing or reading the resist of equipment yourself, then adding up.
There's several solutions to this: Larger meters, additional meters, a new colour overlaying the original blue, etc. So yeah.