The versions are specified in the dependencyManagement part of the parent pom,
as is usual in Maven (you can see them here:
http://search.maven.org/remotecontent?filepath=org/slf4j/slf4j-parent/1.7.5/slf4j-parent-1.7.5.pom).

If Ivy fails to honour the dependencyManagement part of a parent pom that's an
Ivy bug.