Unfortunately with rules, InfoPath treats repeating nodes with an all-or-nothing attitude. There is no built-in filtering for a rule to specify a particular repeating node to update, rather they are all updated. This is unfortunate behavior in my opinion.
Some users have reportedly modified the manifest.xsf file to manually add in a filter to the rule xpath. The other option is to use Qdabra qRules (it's either 1.8 or 2.0 that gives the ability to get around this--not sure which) or write your own code.