How are (Lowther) phase plugs aligned?

I got curious when I came across this statement somewhere on the internet as I assumed phase plugs are centred with the cone. Perhaps there is an advantage in having a small offset, for the same reasons one offsets a driver in a baffle. Anybody got any additional insight to share ?

“I recall, however, that Scott Fraser said that the phase plug mounts eccentrically in this driver, which made aligning the cone especially difficult. “

from: High Efficiency Speaker Asylum