The seal on a binaural head is likely a bit better than you'd get in real ears so inflates the numbers a bit. However, it's still expected to have better measured bass response from the IE phones despite this.
But, the frequency response doesn't tell the entire story in terms of what you 'hear'. For example you don't 'hear' much under ~60hz. What I mean is that frequencies below this are mostly not picked up by your ear drums but rather other senses feeling the pressure waves and your brain is wired such that it get's mixed into your perception of sound. The larger driver in CA headphones tends to activate the 'feeling of bass' more than in-ear headphones which can lead to the perception of louder-bass despite measurements favoring in-ear.
This effect tends to be more significant if you're listening to music with a lot of sub-bass, EDM for example.
But, the frequency response doesn't tell the entire story in terms of what you 'hear'. For example you don't 'hear' much under ~60hz. What I mean is that frequencies below this are mostly not picked up by your ear drums but rather other senses feeling the pressure waves and your brain is wired such that it get's mixed into your perception of sound. The larger driver in CA headphones tends to activate the 'feeling of bass' more than in-ear headphones which can lead to the perception of louder-bass despite measurements favoring in-ear.
This effect tends to be more significant if you're listening to music with a lot of sub-bass, EDM for example.