Hi Andee,
When the x-axis is numeric, the x-coordinate of the plotted point will be exactly its value above the axis. So if month is 4, the point will lie above the 4. If two points have the same value for month they may overlap if their y-values are close.
When the x-axis is categorical, each category gets a bit of space for its points and the points get stacked (horizontally in this case) as needed within that space. So two points with the same month and rainfall will be offset from each other horizontally.
I hope that explains the difference!
Bill