As I understand from explanation, Good Answer is awarded for an answer with score 25+. Guru is awarded for an answer with score 40+ and only if it is accepted. Is it normal that one of two badges of the same color (both silver) is strictly "better" than another, or am I reading something wrongly?
On SO, 8,858 Good Answer badges have been awarded while 1,093 Guru badges have been awarded.
With so many different badges, they are broken down into 3 difficulty ranges (bronze, silver, gold). Just because two badges are both silver doesn't mean the difficulty level of getting them is the same. However, the idea is that the "easiest" gold badge is more difficult to get than the most difficult silver badge; whether or not that works out in practice is open for debate.
In a word, Yes. Guru is strictly harder to attain than Good Answer. At the same time, Civic Duty is easier than that (just need to exhaust all votes over the course of 10 days), and Enthusiast is circumstantially even easier. There's a lot of variance in difficulty within one tier of badges, the main distinguishment is that golds tend to be harder than silvers, and silvers harder than bronzes. Another trend is that you general receive golds after silvers, and silvers after gold.
This is not always the case, of course. Some random statistics on oddball badge difficulty in comparison to the other tiers.