regardless of who makes them, you should check whatever you decide to buy and make sure its true..
the way you can do this if anyone does not know, is you place the level against a vertical object, say a wall. make a mental note of EXACTLY where the bubble is, (say the top window) in reference to the closest line. ( its a hair to the left, or its 1/8 over the line....) after you have a reference, then you turn the level around, so your looking through the opposite side of the same window.. so in other words, if you were looking through the top window, you're turning the level so that same window is still on top, but now you're going to place the opposite edge of the level against the same wall, in the same spot.. take note of where the bubble is, it SHOULD be in EXACTLY the same spot.
you can repeat this process for every window/bubble of the level. some may even have adjustments to fine tune the readings.. depending on what you're using the level for, it may not be such a big deal if the things is off by 1/32 or 1/64 - but remember, that's only over 4 feet..