The -20% may be type-restricted to only certain types of buildables. You'll have to peek into the XML to check that, though. This is a known UI incompleteness:
- Tooltips do not display any type restrictions on buildable cost modifiers.
Just looking at a "-20% Manufacturing Costs", the player cannot tell which subset of buildables will get the -20%.
c.f. Industrial Specialization, which has said since Alpha 0.31:
- -5% Manufacturing Cost <= this one is for social (improvements)
- -5% Manufacturing Cost <= this one is for military (shipyard)
It's not ambiguous ... it just looks that way to casual players 
