There cannot be more than 128 bits entropy in BIP39 12-words. There are not enough bits to represent it.
If you have selected each word manually and randomly and you have 12 words then each word represents 11 bits which makes the total 12*11=132 bits.
Yes, it is 132 bits, but only if there's no checksum or required version.
It very much affects the entropy, since 255 (or 4095 in segwit case) possibilities are rejected (plus the valid BIP39 ones, about one in 16, another loss of additional 0.0931 bits of entropy). You end up with smaller pool of possible seeds, hence smaller entropy.
It seems that entropy is a very tricky subject for many people. I'll give an example. Let's have a hypothetical seed generator, which starts randomly, and increments until it reaches only one specific seed. This is exactly 0 bits of entropy. If the generator stops when it reaches one of 2 seeds, we get 1 bit entropy. If an attacker has no information about these seeds, then he has to scan the whole 256 bit range (or whatever size it is in this case).
So, valid electrum seeds do have less entropy - 123.9 bits for standard, and 119.9 bits for segwit. That doesn't mean it's much easier to crack versus BIP39. If my calculations are correct, it's about twice harder to find a valid Electrum segwit seed versus both Electrum standard and BIP39. (if we are given an address to compare to)
In information theory, the entropy of a random variable is the average level of "information", "surprise", or "uncertainty" inherent to the variable's possible outcomes.
Certainly only one in 2
4 seeds are valid for BIP39, in 2
8.09 for Electrum standard, and in 2
12.09 for Electrum segwit. Hence the entropy is lower.
One might argue, that the attacker sees 132 bits of entropy, since nothing is certain for him. Then this is true for BIP39 as well, although it is generated using 128 bit entropy. Looking the other way if one insists BIP39 to have 128 bits entropy, then Electrum standard has 123.9, and segwit 119.9.