This is exactly how xAxis.ordinal
By default, it is set to true and what it does, is that it makes the spaces between the points exactly the same. So If you have one points on 16th, 17th and 20th of September, the spaces on xAxis between them will be equal. That's why there are different distances between xAxis ticks.
If you turn xAxis.ordinal to false, the spaces between points will be equivalent to how much they are really apart from each other, also the spaces between xAxis ticks will be exactly the same. Check out the demo below:
Let me know if you have any further questions!