So 100cm3 of 0.05 mol/dm3 is the mixing of 100cm3 of water and 5cm3 of sulphuric acid

When you prepare a solution by dilution, you add the calculated amount of concentrated acid (in your example, 5 cm³ of 1 M sulfuric acid) to some distilled water in a volumetric flask. Then, you add more distilled water until the total volume of the solution reaches the 100 cm³ mark on the volumetric flask.

So, you are not mixing 100 cm³ of water plus 5 cm³ of sulfuric acid to get 105 cm³. Instead, you are diluting 5 cm³ of concentrated sulfuric acid to a total volume of 100 cm³ with distilled water.