Porting Guide and Statement made about Char Vs VarChar

[Scott Johnson <sjohnson@xxxxxxxxxxxxxxx>]

http://www-03.ibm.com/servers/enable/site/db2/porting.html

I have had a read thru the "DB2 for i5/OS porting guide: MySQL to System i 
platform" and found a very interesting statement made about when to use 
Char versus VarChar. Page 11 has this tidbit:
"The CHAR data type must always be used for columns of 40 bytes or fewer."

Why would IBM make such a strong statement that we use a Char when the 
field is less than 40 bytes in length?  Would it still be the case when I 
have a 35 character field that over 75% of the time only contains 15 
characters?  What about a 10 character field that is mostly only has data 
that is 4 characters in length?  I realize that with a VarChar there are 
the bytes tacked on to contain the length, but I must be missing something 
someplace.

Can someone with some deep dark knowledge of the internal workings of DB2 
for i5/OS give me some info on why IBM would say this.

Thanks,
   Scott