Synergy calculations (The Witcher 3: Wild Hunt)

Don't understand shit. In short. Noticed a strange thing. Let's say a character without a single spell, no mutagens, the power of signs (for example) 40%.
Insert the mutagen 10%. The power of symbols grows by 10%. A total of 50%. Everything is logical.
Synergy is inserted in the slot where the mutagen no, the power of symbols while not grow on 5%. She is exactly 50% (values for each sign of the same does not change, there inego not changed). Why?
But if you stick to the mutagen 3 skill, which will give 60% instead of 40% (from 3 skills, and a mutagen), it will be added in the amount of 40% to 55% (not 60%). How this fucking works? Has anyone checked? Maybe this is a bug? Is there a fix? XS at all. 1.32 version of course.

Make it easier to understand. The mutagen is inserted in the slot without the skills without synergies displays +10% and +10%, and displays synergy +15%, but gives as well as without it +10%.
PS. With red mutagen too.