Identical numbers do not guarantee an identical cup, and the reasons are mostly physical.
Bean age
The biggest one. Coffee changes measurably every day of its life. Days 5 to 10 behave differently from days 25 to 30 — degassing, solubility and grind behaviour all shift.
Expect to nudge the grind through a bag. That is normal.
Humidity
Coffee is hygroscopic. On a humid day it absorbs moisture, grinds slightly differently, and extracts more slowly.
This is why espresso shots run long in wet weather with no other change.
Water
If you use tap water, its mineral content changes seasonally and with supply. That changes extraction.
Filtered or recipe water removes this variable entirely, and it is the fix people notice most.
Grinder drift
Burrs move slightly under load, and a burr set that has not been re-zeroed drifts over months. Retained grounds from a previous coffee mix into the next dose.
Purge a few grams after changing settings or coffees.
Temperature
Ambient temperature changes machine warm-up and heat loss. A cold kitchen in winter genuinely brews cooler.
Your palate
Real and often ignored. What you ate, whether you have a cold, and time of day all change perception substantially.
What to control
Weigh everything. Use consistent water. Buy in sizes you finish in a month. Accept the rest.
Last reviewed 8 August 2026