Block border/padding behaviour

I am trying to produce a block with the background-color filled but with 
content leaving a '12pt' gap (filled with background-color) on the end edge.

I expected to achieve this by specifying a padding-end of '12pt' and 
text-align='right'.
This works perfectly in HTML + CSS.

However both FO processors I have tried (FOP + XEP) ADD the padding 
space  to the block width (which I have tried both specified and 
unspecified)
and so my content stays where it would be without the padding attribute 
but my block is suddenly '12pt's wider!

E.g.
I get:
 ---------------------------------------
|                    content            |
 ---------------------------------------
whereas I want:
 ---------------------------
|        content            |
 ---------------------------

Am I mis-interpreting the specification
- or are these processors both implementing the specification in a 
(currently) non-standard way
- or is there some subtle interaction with content-width that I should 
also be specifying.
(FOP does include a note that padding 'cannot be used to create extra 
space' - whatever that means.
XEP does not have any qualifications of the behaviour of block + padding.)

OR Any alternative suggestions as to how to achieve the above effect?

(I can do this by including an enclosed block with a right margin / 
end-indent
- but this vastly complicates my allocation of styles / layout attributes
(which are triggered by a '@class' attribute - so I can match HTML output)
as some would apply to the inner-block and some would apply to the 
enclosing block.

Any clarification / help appreciated.