- Parson was identified as a Hippiemancer (undefined) by Janis
- He sees jokes and references in everything, that native Erfworlders do not
- Signamancy is the school associated with Fate
- He made street signs for the new GK
- Date-a-mancy is also a school of Hippiemancy
- Date-a-mancy tries to understand "the Numbers which underlie all action. Date-a-mancy tries to quantify the intangible[...]" (Duncan Scone, LIAB Text 11)
- Parson regularly thought up battle-sims of unlikely combinations (non-archers pitching bricks in the courtyard, tunneling dwagons vs wheeled ships)
- Signamancy is the school about how things look, and interpreting what they mean.
- Date-a-mancy is the school of how things work, and understanding the mechanics behind them.
Furthermore, Parson doesn't care about how things look. Beyond the street signs, he's done nothing to modify GK's livery, cities or themes. He hasn't designed any clothes or buildings, any new flags to stoke Stanley's ego. He has not painted anything red to see if it goes faster.
Parson doesn't spend his time looking at things and asking "What does it mean?" Parson instead obsesses over "What would happen if X did Y?"
The non-archer with a brick, for example. Sizemore & Maggie, when confronted with the idea just shrug and go "You'd probably miss." Parson takes the time to determine exactly how probable it would be. (1 in 5400 of hitting) And then he starts thinking about "What if the non-archer was buffed with Luckamancy?"
Ultimately, it comes down to this: Parson is a powergamer, not an English Lit Major. He's interested in pinning down exactly how the rules, work, interact and conflict so he can exploit them for loopholes and crazy plans, not in analyzing every aspect of the world to write a 130 page dissertation about how Everyone is Jesus in Purgatory.
Honestly, I think the only reason it's date-a-mancy is because it would be TOO obvious if it were actually called data-mancy.