Yes, it is probably due to a special architect, and the window delivery was delayed. Otherwise, the building is also more individual, so the craftsmen need more time to go through the plans.
Sometimes it only takes one trade that slacks off, then you can forget a set schedule.
For us, it was the roofer, who definitely cost me about a month...
What I like about the general contractor/main contractor is that everything usually meshes together because it is coordinated. With our neighbor, everything just flies by - they have long since overtaken us but started months later.