You would have noticed in the last couple of tasks that there was a minimum and maximum block width that could be set. This setting enables the Moodle themer to have control over the block's column widths in such a way that the columns will stretch to a maximum width and contract to a minimum width, so that the block's contents do not get squashed if the screen size is small. This will work only if you have a full-screen liquid layout (relative), and it is a good way of making sure that your blocks remain within a certain size tolerance.
In the last exercise, you set both the minimum and the maximum block sizes to the same values because you currently have a fixed width layout.
In the next exercise, you will be setting both the minimum and the maximum block widths to different values. In order to see this work properly, you will have to change your theme from a fixed width layout back to the original liquid layout.