...I think on TZ some time ago, in a fairly heated discussion, if I remember.
Someone had noticed something similar to your observation, that the "4" on the Portuguese dial did not appear perfectly equidistant from the "3" and the "5", whereas the 8 appeared "correctly" placed between the 7 and 9.
A lot of back and forth ensued (ranging from parallax to weighting of numbers above/below their equator to placement of numbers on radial lines from the center). Approached religion after a while.
I'll go with magellan on this one. Dial is designed as a unified whole. Arabic numbers in general present problems for typographers because their script origin does not readily lend itself to regular, linear placement as opposed to Romans. Dial designers wrestle with these issues and come up with what they feel is the best overall solution.
Go ahead and fall in love with your IWC all over again 
-Tom