possibly in an extremely lean/emaciated and tall sample.

Chronic undernutrition develops only with a time lag following periods of deprivation and acute undernutrition. Thus, if you assessed a population only recently exposed to severe food shortages, you would expect to find what you describe. Linear growth has not yet been affected but body mass has.

This may occur in normal situation where the GAM is below the emergency threshold levels and where stunting rate is also below the intervention threshold. So When there is acute crisis, disaster or an outbreak, so GAM will be rapidly increased while stunting is static and needs more time to be obvious. so the GAM/Stunting ratio will be increased in this case.
Best

The question about the prevalences of stunting and wasting is important: stunting occurs (and is reversible) at any age as long as growth has not finally come to an end. Wasting occurs often with a seasonal pattern, after catastrophies, and in association with diseases, e.g. TB and AIDS. Therefore, different 'rates' are to be expected.

In a study we could show that improving breastfeeding pattern is an effective approach to prevent stunting in the infant and young child: (free online available) Exclusive breastfeeding and its effect on growth of Malawian infants: results from a cross-sectional study. Kuchenbecker J et al. (2015) Paediatr Int Child Health 35(1):14-23.