You’re right. Looking at the source code (https://github.com/tytso/pwgen/blob/master/pw_phonemes.c#L59), the algorithm seems to be rather complicated, so I can’t say what the exact number of bits is. But we could certainly calculate an upper bound:

7 letters a-z which are either upper or lower case, plus an unknown digit at an unknown location, gives:

(26+26)⁷×10×8 = 82245736202240 possible passwords, giving log2(82245736202240) = 46.225006121875005 bits. Subtracting the bits for the 8×20 choices of passwords gives

log2((26+26)⁷×10×8)−log2(8×20) = 38.90307802698764 bits as an upper bound of the security of a password chosen by a user from the default output of pwgen(1). This is a bit more than the 34.7 bits I first thought it was, but not much more. And this is an upper bound; since I can see that the source code does not choose each character completely randomly and does, as you say, seem to prefer lower case letters, the correct number of bits is guaranteed to be lower than 38.9.