You've based your query on one very large assumption (I assume .. ha .. maybe I am wrong too!). And that is, that the factory map is correct.
Now, we know that is not so. Take a look at the stock Euro map for example, it's full of -ve number indicating that in certain ranges the bike is running too rich.
Translate this to a pipe, and yeah, you may still have to lean it out rather than richen it up. Just because you put a pipe on and let a bit more flow through does not always translate to more fuel being required to be injected.
Some of the late 90s Fireblades actually went *down* a jet size or two when you put pipes on them.
I guess what I'm trying to say is that just because you put a pipe on, does not mean you always need to richen the map up all the way through.
"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." Albert Einstein
'85 GSX250S -> '96 ZX6R -> '98 ZX6R -> '00 ZX6R -> '03 CBR954RR -> '04 ZX10R -> '02 ZZR250 -> '06 ZX10R -> '08 KTM 990 ADV