When the fabricator crafts items that can be made in a Foundry (for example Foundries) it doesn't benefit from the Productivity bonus you would get if you crafted them in an actual Foundry.
I tested it by putting exactly 2 sets of ingredients for a Foundry in the storage and it could only build 2 of them.
If I put the same ingredients in a Foundry I would get 3.

I thought about that, but decided against adding it. There are a bunch of annoying problems related to allowing productivity in this context. For instance, with 50% productivity some buildings would cost 6.6666666..... red circuits or whatever, which would lead to inaccuracies and look ugly in tooltips.