Do your columns have lines or borders? Try removing any borders.
This is what I've observed. I don't have the cure for your problem.
With an embedded chart, column distribution can get a little off when you change the size or change the height to length ratio of the chart. Two columns may appear closer together than a different pair columns. I notice this mostly when I reduce chart dimensions. The apparent distortions seems be worst with chart sizes smaller than 3-inches vertically by 3.25-inches horizontally
I'm using a United States locale. Chart sizes are given in inches—my default charts are always 3 inches high by 5 inches wide. Can you lessen the gap by increasing or decreasing the chart size?