I can't seem to reproduce your issue using the following:
1.) Add XY data, using Excel sheet as source to make an event layer (42 points).
2.) Save and close the mxd.
3.) Add 3 new points to Excel (45 points).
4.) Save and close Excel.
5.) Open mxd. There's now 45 points.
In your event layer, try clicking "Reload Cache" under the attribute table options.
I'll add that event layers are quite mysterious to me. The only thing I ever do with them is immediately export to a permanent feature class.