why does ps |grep sometimes not return itself?

[Eitan Adler]

Why do I sometimes see the grep in ps's output and sometimes not see it?
[eitan at AlphaBeta ~ ]% ps aux|grep Me
eitan 96325  0.0  0.0  1856   724   5  RL+  10:14AM   0:00.00 grep Me
[eitan at AlphaBeta ~ ]% ps aux|grep Me
[eitan at AlphaBeta ~ !1! ]%

-- 
Eitan Adler