Patio roof rafter spacing
We use 19.2" oc joists (5 spaces for 8' plywood) when we use Trus-Joist open web joists. They are so expensive that keeping size down and spacing maximized saves costs. (True-Joist has a software program to figure this out...)

Maybe the alternating spacing allow fasteners to secure the corrugated roofing on the "down" curve of the corrugated shape. 

I think this old-time carpenter was ahead of his time.