400 + 100 = 600?
Not in my dairymaid math *grins*
On a more serious note, the issue will become worse all the time, except for the short period after saturating the average with 500 rounds (expulsion of high scores).
After that, the difference between new and expelled scores will shrink continuously, resulting in ever smaller drops. When - for example - a 63 replaces a 64, the average will drop by 1/500 =.002, invisible in a two digit average display. You will need five of those to safely pass a rounding line, eventually modifying the display.