Why do gases occupy different volumes in eudiometry?

[Logic hunter]

I know how to solve problems of eudiometery but i don't understand a really basic thing ie how come gases occupy different volumes in an eudiometery tube. (For eg: 15ml of hydrocarbons with 320ml other gases). Gases occupy entire volume of the container, right. So shouldn't all gases occupy the same volume equal to the volume of eudiometery tube?

 

[Borek]

When you mix 1 L of oxygen and 1 L of nitrogen under STP you get 2 L of a mixture. You can say each gas occupies 2 L and its partial pressure is 0.5 atm, you can say each gas occupies 1 L and has the pressure of 1 atm. Technically only the former is 100% correct, but the latter makes calculations and thinking about the mixture composition easier. Kind of a mental shortcut.