The impression of sweetness in a wine isn’t just down to how much residual sugar and acidity it has. The following list of factors can also play a role:
Ageing in contact with oak can release sweet-tasting compounds into the wine. It can also impart flavours that are associated with sweetness, such as naturally occurring vanillin, or caramel or coffee flavours produced from toasting the oak over a flame.
The flavours of very ripe grapes – overripe, jammy or dried fruits – can prompt us to think of the sweetness that would usually accompany these flavours in nature, even if the wine which contains these flavours has been fermented to dryness.
Alcohol can taste sweet (as can glycerol), so a wine with a high alcohol content can taste slightly sweet even if it contains practically no residual sugar.
The lower the serving temperature, the less sensitive the palate is to sweetness. A very sweet wine can taste cloying at room temperature, but more refreshing and drinkable when chilled.