Q:

Why does SSL certificate for *.example.com matches example.com as well?

Explanation:
If one issues a certificate for example.com, then it can be used for https://example.com
For https://www.example.com one would have to provide another certificate which will match the sub domain.
Apparently, certificates for *.example.com are common, so a single certificate covers all you will ever need under your domain, e.g. mail.example.com, www.example.com, account.exmaple.com etc. etc.
However, I would not expect *.example.com to match example.com and yet, it does! And I wonder why.
Is it part of the standard?
Is it just a common browsers convention?

A:

For https://www.example.com one would have to provide another certificate which will match the sub domain.

This is not exactly so. CAs usually issue a certificate valid for both example.com and www.example.com

However, I would not expect *.example.com to match example.com and yet, it does! And I wonder why. Is it part of the standard?

Check if the certificate contains SubjectAlternativeName extension with just "example.com" in it. This is the most likely answer. 
RFC 2818 (section 3.1), as I read it, doesn't cover example.com by *.example.com , but this can be my interpretation. 

