As is well known, the agency rules are repeated in Houses of Hermes:Societates p140-144 and Lords of Men p22-25.
I ran across a difference
When the principal earns Adventure experience, he simultaneously earns an equal number of Agency experience points, which are spent to increase the agent’s Bond score using the same progression as Abilities (see the Advancement Table, ArM5, page 31). LoM p25
Once the story has been played out, the storyguide awards Adventure experience points in the usual fashion, and an equal number of agency experience points are applied directly to the strength of the Bond, using the same progression as Arts on the Advancement Table (ArM5, page 31). HoH:S p144
If you use the agency rules, how do you deal with this difference? Is there any official word - I'm guessing not.
I suppose this depends on how quickly you want to build agents in the game. HoH:S is clearly the rule which was written first.
The example in HoHS makes it seem clear enough, so stories easily get you enough to boost you up a level. Actually, the HoHS rules make it look like spending money is enough to get you up to five xp towards this, to a max bond strength of +3, so the rules look like recruiting an agent is the hard part, bribing them gets them on side, and then doing adventures that help them out make them fanatically loyal.
After reading into it further I think the key is that LoM makes it clear that it is trying to simply repeat the rules for use in more general circumstances than what they were first written for, and as such I believe that the switch from arts advancement to abilities advancement was an error or perhaps a house rule the author forgot was not RAW and was missed by the editor.