I'm not positive, but they both may be correct. The base capture rate for raid bosses is probably lower than for Pokemon encountered in the wild. It's not exclusive to T3 raids either; Snorlax has a base capture rate of 4% in raids but 5% otherwise. The 20% decrease is consistent across all the examples I've come across, so it seems legitimate. If I were to guess as to why, it would be to compensate for the fact that raid bosses cannot flee, but who knows?
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ANSWERED: Tier 3 boss base capture rates
For Machamp, Alakazam, Gengar, and Arcanine, the Pokemon pages list them as 10.0% while the Raid Boss List page lists them as 8.0%. For the Eeveelutions, the Pokemon pages list them as 12.5% while the Raid Boss List page lists them as 10.0%. Which are correct?
I'm not sure if there are any other similar discrepancies, I just happened upon these.
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Answers
Except for some pokemon, the capture rates match. Venusaur, for example, shows 5.0% on both the raid boss page and on the actual pokemon page.
Also, base capture rates are always the same despite level. What changes is the capture multiplier. For example, the capture multiplier for a level 20 pokemon is 0.597. For a lower level pokemon, the capture multiplier is lower. The capture rate is then determined with the formula:
% chance to catch =
(base capture x multipliers)/(capture multiplier x 2)
