I think we are forgetting however, that 138 is still quite fast, and almost sounds allegretto. Saying that it's lento because if it's in one, it's around 40, is an oversimplification. Think about it this way, The minute waltz should be quite fast, perhaps Dotted half=90ish. However, according to everyone's metronome, that's only an andante. However, the waltz should be allegro, which means (according to your simplification) that it should be at least 120 to the dotted-half.
I think the key point here is that it's a waltz. Waltzes are meant to be danced to and thus cannot be too slow to the point of where dancers are forced to be suspended in awkward positions for long periods of time. Since this is a waltz to be danced to, one should always keep the flow going, however, Chopin does write Lento. I think that what he means by Lento is that it should be as slow as possible while still maintaining the waltz feel.
Someone correct me if I'm wrong?