­

[pies]

­

[Sketchee]

It's not odd.  There are somewhat different skills needed in memorizing these types of passages.  Some of what you use to practice chordal piece can be applies to single notes.  Look at the intervals between notes as if they were chords.

It's frequent that people have trouble memorizing pieces that they can easily read.  That's because it takes less thought to read and so they have fewer references of memorization.  That's when we must scan for reference points like intervals, chords, and any pattern you can find that will help you remember.  Look at it in many ways and that'll help prevent memory slips.