In all likelihood, the 18.5" measurement is at the top of the seat at shoulder level, while the 'standard' measurement is armrest-to-armrest. 18.5" width at the armrests is realistic for an 8-abreast 787, not 9-across. To put things in perspective, 18.5" width between armrests at 9-abreast is about as wide a seat as is possible on a 777, and a 777 is 16" wider than a 787.

In other words, the math doesn't add up.