Two other factors are stamina and context. Most reading tests by necessity use very short passages (the ones measuring lexile are longer tests, but still based on very short passages). Some students score lower on the assessments relative to the books they read, because their comprehension depends on use of context and there isn't enough for the short passages, which are often incomplete portions of stories. Other students do very well on assessments relative to what they actually read because they have difficulty sustaining attention and/or synthesizing information over longer (book length) texts.
I do think that, in a way, comfort is a part of reading ability, because it speaks to the application of reading skills in everyday situations. On the other hand, there is also interest. If you think about our recreational reading choices as adults, they are probably rarely at our maximum ability levels.
