Why use 甘い in this context? 「今朝のみそ汁はちょっと甘い。」

Question

Why use 甘【あま】い in this sentence and not 薄【うす】い?

今朝【けさ】のみそ汁【しる】はちょっと甘【あま】い。(- 塩分【えんぶん】が少【すく】ない)

The English translation in the Somatome book is ‘The miso soup is rather mild this morning.‘

Does it have something to do with 方言【ほうげん】? I'm currently living in Gunma and my Senpais told me that they're not using 甘【あま】い in this context. みそ汁【しる】は薄【うす】いとか。 I've read a post in Yahoo Japan about it and they have different answers. My understanding in this sentence is that the miso soup is not tasty/salty, that's why the English translation of ちょっと甘【あま】い is "mild".

Answer 1

甘い does mean "mild" or "not spicy" when used to describe the spiceness of curry and such, but it's not common to use it to describe the taste of miso soup. But 甘い味噌汁 not an impossible expression, either. If hear it, I'd probably assume it means either "sweet/sugary" or "thin (not salty enough)", and try to guess the intent from the context or the ingredients first. I may not bother asking back unless I really need clarification.

辛い has some historical or dialactal meaning (does 辛い means both spicy and salty?), but I'm not aware of regional variants around 甘い.