Two Tamil methods compute the same porutham — but from two different almanac traditions. Here is the difference, and why juno.date sets Vākya as the default.
The traditional Tamil almanac lineage. Planetary positions come from the classical vākya — mnemonic Tamil verses handed down over centuries — the same basis the temple and village panchangam has always used.
It is the purely Tamil stream: unbroken tradition, computed in the Tamil way, with none of the later external corrections mixed in. For families who want the reckoning their elders and the kovil almanac follow, this is the authentic one.
The modern, astronomically corrected method — Drik-ganita, positions matched to observed astronomy — paired with the pan-Indian rule conventions now common in printed almanacs.
Those conventions simplify some older Tamil distinctions — for instance they merge the ox and cow yonis into a single "cow" — which is why the two can score a couple differently even when the birth data is identical.
Here is the important part: the difference is almost never in the chart. The star and rasi usually come out the same either way. It is the rule tables that differ. Comparing one real couple across the two engines, the core poruthams — Rajju, Dina, Nadi, Rasi and Stree-Deergha — agreed completely; the scores diverged only on these conventions:
Yoni (ox vs cow). Vākya keeps ox (எருது) and cow (பசு) as separate animals — and a tiger's enmity is with the cow only. The modern table merges both into one "cow" (Hindi go / மாடு), so a groom whose star is an "ox" can pass yoni under Vākya yet fail under the merged table.
The twelfth porutham. Vākya uses Vriksha (tree); the modern uses Varna — a different twelfth check entirely.
Stree-Deergha limit. Vākya asks the count to be strictly beyond 13; the modern accepts 13.
Gana & Rasi-lord. Slightly different friend/neutral tables, so a borderline pair can read "present" in one and "absent" in the other.
None of this is an error on either side — each table is internally consistent. It is simply two schools counting by their own rules.
Two reasons, both Tamil-first: granularity and purity of tradition.
Granularity. Vākya keeps the finer classical distinctions — the separate ox and cow yonis are the clearest example. Where the modern table collapses them into one animal, Vākya still tells them apart, so its verdict carries more of the detail the elders actually weigh.
Purely Tamil. Vākya is the self-contained Tamil reckoning — the classical vākya almanac and the traditional Tamil porutham rules (Vriksha as the twelfth, the ox/cow yoni, and so on) — without the later pan-Indian conventions layered in. For a Tamil match, we let the Tamil method lead.
Thirukanitha is a single tap away for anyone who prefers the modern, astronomically-corrected view — we show it too, we simply do not make it the default. Neither is "wrong": they are two honest traditions.