Imperial Templar cost reduction appears to be bugged

virtus753

Combining cost reduction from Breton (7% mag cost reduction) and Templar (5% universal cost reduction) yields the expected 12% cost reduction for mag skills.
e.g. Puncturing Sweeps goes from 2295 magicka to 2020 magicka (275 = 11.98% of 2295)
This shows the two cost reductions are additive, since a multiplicative relationship would yield a cost of 2027.6, not 2020.

Combining cost reduction from Imperial (6% universal cost reduction) and Templar (5% universal cost reduction) yields only 10% cost reduction for skills, whether mag, stam, or ultimate.
e.g. Biting Jabs goes from 2295 stamina to 2066 stamina (9.98% of 2295)

Individual cost reduction from Imperial and Templar yields the correct values for Jabs: 2157 and 2180, respectively. It's only in combination that the value is not as expected.

Even a multiplicative combination of 6% and 5% would result in a 10.7% cost reduction, not 10.0% as seen in game. (0.95 * 0.94 = 0.893)
additive 11% would result in Jabs costing 2042.55
multiplicative 10.7% would result in Jabs costing 2049.44

So this combination is worse than either additive or multiplicative. It is also inconsistent with the way Breton and Templar cost reductions interact. If the Imperial and Templar cost reductions are meant to be multiplicative *and* the end result is rounded against the player's favor, which is the only way I can see how 5% and 6% cost reductions produce a 10.0% cost reduction, then the Breton and Templar passives should likewise combine to be only 11%, not 12%. Something is very inconsistent here.

Tested with no other sources of cost reduction (no armor, no cost reduction glyphs, no skills that benefit from skill line cost reduction, etc.).
Cost confirmed in CMX combat log, so it does not appear to be a tooltip/UI error.