I think the reading is an indication of "close reading," i.e., can they interpret text and use it to make inferences about what's happening or going to happen in a story.
English is grammar.
Since I know nothing about Explore, take this with a grain of salt.... but I can say that inferencing and related skills are still pretty weak in grade 4s: developmentally, reading what the text doesn't say is a skill that grows over time and requires explicit teaching for many kids. For example, my DS's grade 6 English teacher, in a gifted class, has explained to me that a significant portion of the kids in the class struggle hugely with inference and making textual connections, and this is normal.
So based on the many conversations I have been having with grade 4 and 6 teachers this year about the age of readiness for these specific kinds of tasks, it makes sense to me that so many of you report younger children doing much better on the clear, rules-based part of the Explore than the inferencing et al bits.