Look at these crochet needles on my spinning cloth. All are size 2,5 (mm I presume). If you don’t swatch very carefully when using a written pattern you may get very far from what you had planned, even when you use the suggested yarn. The three needles with the same thickness in the shaft have different hooks, so the result will be different if you change your needle to another in the middle of a project.

Swatching, swatching, that’s the life of a crocheter…

Just for fun, have a look at different ways of sizing and numbering hooks. There are plenty of charts on internet.