There is a feedback by which SAI causes stratospheric heating which then changes the circulation patterns, such that the eventual aerosol distribution differs from what would be expected for a passive tracer.
Metric
The spatial distribution of SO4 aerosol differs from a hypothetical passive tracer by a sufficient magnitude to cause a large difference in optimal injection locations for a balanced strategy achieving 0.5°C cooling
Uncertainty
Models generally suggest this is a small feedback - for example in GLENS, which has much more injection and cooling than our scenario, the spatial pattern of feedback-controlled injection is fairly constant over the simulation
Decision relevance
This could be accounted by changing injection latitudes over time, and even if the sub-optimal injection locations at the original location would not necessarily be strongly less effective.