Different labs, even different labs within the same franchise, often use different tests, instruments, and laboratory personnel and each can introduce a small amount of variability. Most labs though have a fairly reliable degree of accuracy and precision. For accuracy, consider a dart board with a dart in the centre. For precision over a number of tests using the same sample think of a dart board with a cluster of darts on the board but not necessarily in the centre. What you would prefer obviously is that a test(s) was both 100% accurate and precise, clustered darts in the centre, but it just isn't possible. Lab results tend to be accurate and precise enough, especially if you use the same lab all the time. If they weren't, pathology labs would lose accreditation. With most results, the important thing usually is how they change over time rather than the actual numbers themselves.
So, different labs have different ranges because they've factored in their equipment and their methodology, the equipment they do tests with and how they do it.