Peck says of the first reverse that the exergue line is "faintly single" and of the second that it's "distinctly double". He doesn't go on to explain this further, but the implication is that the single line isn't clearly single, but - as others (I believe) have said - consists of two lines so close together that they appear (especially on less than perfect condition) to be a single line. I'm not sure what condition the 4th penny above is in but it MAY illustrate this 'grey area'?
My penny is all but BU so if it is the second reverse, the double lines would be absolutely clear - they're not!