Negative offsets in variable substitution

[Bob Dowling]

Version of bash:        2.05b.0(1)-release

Problem duplicated on AMD64/SuSE9.1 GNU Linux and Apple G5/MacOS 
10.3.6.  Both ship wit the same version of bash as given above.

"man bash" says in the Parameter Expansion section under Substring 
Expansion (${parameter:offset:length}) that "[i]f  offset  evaluates  
to  a number less than zero, the value is used as an offset from the 
end of the value  of  parameter."

However, experience suggests otherwise:

$ xyz='abcdefghijkl'
$ echo ${xyz}
abcdefghijkl
$ echo ${xyz:3}         # Positive offset - OK
defghijkl
$ echo ${xyz:-3}        # Negative offset - Not OK
abcdefghijkl

I don't know whether this is a bug in the manual page or the bash 
program.  Or in my understanding of the manual page.