Employees on lower salaries get a special reduction that cuts their IRPF a lot: the work-income reduction (reducción por rendimientos del trabajo).
How it is worked out
It depends on your net work income, the gross salary minus Social Security:
Up to €14,852: the reduction is €7,302.
Between €14,852 and €19,747.50: it shrinks gradually.
Above €19,747.50: no reduction.
Its effect
On €18,000 gross in Cataluña the net is €16,206 and the year's IRPF just €624. Without the reduction it would be much more.
The cliff that is not there
Because the reduction shrinks gradually, earning a little more never leaves you worse off. But where it shrinks, each extra euro is taxed at a higher marginal rate than usual. See marginal and effective rates.
More help for low salaries
Salaries near the minimum wage also get a specific deduction that can bring IRPF down to zero.
The Basque Country has its own bonus and Navarra has no such reduction: see the foral system. Check your case in the calculator or on the €18,000 gross page.