Answer: Dot gain!
Dot gain, or tonal value increase, is caused by ink spreading around halftone dots, which causes printed material to look darker than planned for.
Several factors can contribute to dot gain. Different paper types, for example, have different ink absorption rates. Uncoated papers can absorb more ink than coated ones, and thus can show more gain.
Dot gain is neither good nor bad, but simply a normal result of the printing process that should be taken into consideration during the creation of the design file, the choice of papers, printing process, inks, etc.