The flexible content block widget automatically adjusts the widths of items to fill the available space. So two items or three items in a single row will fill the same space. This means that in rows of two items each block is wider than it would be in a row of three.

When using some odd number combinations of items eg. 5, 7, 11, you might want all your items to maintain a uniform width, (rather than each on the row of two being wider than each in the row of three).

You can override the default automatic adjustment and make all the items of equal width in the widget by selecting On in the Fixed column width option. 

Screengrab of the Flexible content widget