Why does my grinder give a different dose each time?

Almost every grinder that doses “automatically” doses by time, not by weight. It runs the motor for a set number of seconds and hopes.

Why that drifts

Bean age. Fresh beans are denser and moister and flow differently from the same beans three weeks later. The same three seconds delivers a different weight.

Bean type. A light roast is denser than a dark one, so the same time gives a different mass.

Hopper level. A full hopper pushes down on the burrs and feeds faster than a nearly empty one.

Grind setting. Finer grinding is slower, so the same time delivers less.

Those add up to variations of a gram or more, which on an 18 g (0.6 oz) espresso dose is over five percent.

The fix

Weigh the dose. A 0.1 g (0.004 oz) scale costs very little and removes the variable permanently.

If your grinder has a timer, use it to get close and then trim by weight — grind, weigh, add or remove a fraction of a second next time.

Grind-by-weight grinders

They exist and cost considerably more. They solve this properly and are worth it only at volume.

The related point

If your shots vary day to day and you grind by time, this is very likely the whole explanation before you look anywhere else.