Why doesn't ClearAll[“Global'*”] remove all variables? [closed]

Consider the following code :

In[19]:= a = 2;

In[20]:= ClearAll["Global'*"];

In[21]:= a

Out[21]= 2


As you see, the variable a hasn't been resetted. Why ? ClearAll["Global'*"] is supposed to clear all user defined symbol : How do I clear all user defined symbols?

closed as off-topic by AccidentalFourierTransform, Jason B., Bob Hanlon, Kuba♦Jun 23 at 20:11

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "This question arises due to a simple mistake such as a trivial syntax error, incorrect capitalization, spelling mistake, or other typographical error and is unlikely to help any future visitors, or else it is easily found in the documentation." – AccidentalFourierTransform, Jason B., Bob Hanlon, Kuba
If this question can be reworded to fit the rules in the help center, please edit the question.

• $\neq$'${}{}{}$ – AccidentalFourierTransform Jun 23 at 15:30
• not ClearAll["Global'*"]; but ClearAll["Global*"]; – Xminer Jun 23 at 15:32