South and Central American beans TEND to be wet-processed and/or meticulously graded, compared to Ethiopians which TEND to be dry-processed and ungraded. What this means is that most Ethiopians (in my experience as well as yours) appear very uneven in the roaster (as well as having differing sizes of beans and some beans - called quakers - which never progress past ~CS5). Weve found that the differences in colour largely even out by the time you reach second crack (ie. no more than 1 CS-point difference, no counting quakers).
FWIW, when we roasted a different Limu, we found that you need to aim for at least CS8 to avoid an overly sharp acidity dominating the cup... this Limu might be different. We loved the Limu we recently ran out of at about CS10 - roasted into rolling second crack, 2-3 mins past the start of SC. It was a fruity choc-fest.
As for whether youre roasting them correctly, tell us how youre roasting them? What kind of FC and SC times did you have for the Ethiopian roasts?