These two rows often sit together on a report and are frequently read as two attempts at the same measurement. They are not. One covers a moment; the other covers a season.
Fasting glucose: one moment
A fasting glucose is the concentration of glucose in your blood at the instant the sample was drawn, after a period without food. MedlinePlus explains why the fast matters: "When you eat and drink, your bloodstream absorbs sugar, fat, protein, and other nutrients in those foods and beverages. These nutrients can then change the levels of various substances in your blood."
So the number is a snapshot, and the fast is what makes snapshots from different days comparable.
HbA1c: roughly three months
HbA1c measures something different — not glucose itself, but glucose stuck to a protein. MedlinePlus: "Glucose in your blood sticks to hemoglobin, a protein in your red blood cells. As your blood glucose levels increase, more of your hemoglobin will be coated with glucose. An A1C test measures the percentage of your red blood cells that have glucose-coated hemoglobin."
The time window comes from the cells themselves: "An A1C test can show your average glucose level for the past three months because: Glucose sticks to hemoglobin for as long as the red blood cells are alive. Red blood cells live about three months."
MedlinePlus describes the result as "your average level of blood glucose... over the past two to three months."
This is why HbA1c does not require a fast in the way a glucose measurement does — what you ate this morning is a rounding error against three months of accumulation.
Why they can disagree
Because they are measuring across different windows. A fasting glucose reflects the last several hours; an HbA1c reflects a quarter. A single unrepresentative morning moves one and barely touches the other.
There is a second reason worth knowing, and it follows from the mechanism: HbA1c depends on red blood cells living roughly three months. Anything that changes how long red cells survive changes what the percentage represents, without glucose having changed. That is a clinical question rather than a reading question, but it explains why the two numbers are not interchangeable evidence.
The "estimated average glucose" row
Some reports print an estimated average glucose alongside HbA1c, in the units used for a glucose measurement. It is converted from the HbA1c percentage by a formula — it is the same measurement expressed differently, not an additional one.
Treating it as a second data point double-counts a single result. It belongs in the same family as the other calculated rows on a report.
Reading them together
Neither number is the more "real" one. They answer different questions, and a report carrying both is offering two time horizons rather than a measurement and a check on it.