Two reports, same laboratory, same marker: 5.6 last time, 5.7 now. Whether that difference means anything is not something the two numbers alone can settle.
Reports print to fixed precision
A laboratory decides how many decimal places to publish for each marker, and rounds to it. That decision is per-marker: creatinine is commonly reported to two decimal places, HbA1c to one, sodium to none.
The consequence is that the printed value is a rounded view of the measured one. Two results printed as 5.6 and 5.7 may have been measured 0.02 apart, either side of a rounding boundary. The report cannot show you that, because the digits were discarded before printing.
Every measurement varies
Repeat a measurement on the same sample and you do not get an identical answer every time. Instruments have a characteristic amount of variation, and it differs by marker — some are very reproducible, others much less so. Splitting one tube of blood and running it twice produces two slightly different numbers.
Biological variation sits on top of that: the same person genuinely differs between mornings, before and after a meal, and with hydration.
So a difference between two reports has at least three possible sources — rounding, measurement variation, and a real change — and the two printed numbers do not distinguish between them.
What this means in practice
A small movement inside the reference interval is usually not informative on its own. The things that make a difference interpretable are the size of the change relative to how much that particular marker normally bounces, whether the same laboratory and method produced both, and whether the direction persists across more than two readings.
That last one is the most useful and the least available from a single report — which is why a run of results says more than a pair.
What this does not mean
It does not mean small changes never matter. Some markers are consequential at small movements, and whether a particular change matters for a particular person is a clinical question, not one this page can answer. MedlinePlus's framing holds: a result inside the range "isn't always a guarantee of good health", and one outside it "may or may not be a sign of a health problem".
The point here is narrower. Before reading anything into a difference of 0.1, it is worth knowing that the printing itself could have produced it.