The EARLIER function by default refers to the row context that immediately preceded the current row context being considered. In the examples used here there have only been 2 row contexts, the outer and the inner. Given there are only 2, when using EARLIER within the inner row context it is always referring to the outer row context. In this case, EARLIER and EARLIEST refer to the same outer row context. If there are more than 2 row contexts, the EARLIER function accesses the previous row context by default (starting from the inner and moving out) and the EARLIEST function refers to the outermost row context. But did you know that EARLIER has a second optional parameter?
Read the whole thing.