Specified strength (f'c)
The compressive strength the designer requires the concrete to achieve, stated at a defined test age. It is a design input, not a prediction of any single cylinder result.
What does specified strength (f'c) mean?
Because concrete strength varies from batch to batch, producers deliberately target an average strength above the specified value so that the statistical acceptance criteria are met. This overdesign is normal and is the reason cylinders routinely break well above the number on the drawings. Acceptance is evaluated on averages of tests rather than on individual results.
In practice
A cylinder breaking above the specified strength is expected; a producer whose cylinders land exactly on the number is running uncomfortably close to the edge.